Optimize common path of Once::doit

Optimize `Once::doit`: perform optimistic check that initializtion is
already completed.  `load` is much cheaper than `fetch_add` at least
on x86_64.

Verified with this test:

```
static mut o: one::Once = one::ONCE_INIT;
unsafe {
    loop {
        let start = time::precise_time_ns();
        let iters = 50000000u64;
        for _ in range(0, iters) {
            o.doit(|| { println!("once!"); });
        }
        let end = time::precise_time_ns();
        let ps_per_iter = 1000 * (end - start) / iters;
        println!("{} ps per iter", ps_per_iter);

        // confuse the optimizer
        o.doit(|| { println!("once!"); });
    }
}
```

Test executed on Mac, Intel Core i7 2GHz. Result is:
* 20ns per iteration without patch
*  4ns per iteration with this patch applied

Once.doit could be even faster (800ps per iteration), if `doit` function
was split into a pair of `doit`/`doit_slow`, and `doit` marked as
`#[inline]` like this:

```
#[inline(always)]
pub fn doit(&self, f: ||) {
    if self.cnt.load(atomics::SeqCst) < 0 {
        return
    }

    self.doit_slow(f);
}

fn doit_slow(&self, f: ||) { ... }
```
update valgrind headers
Add ToCStr impl for &Path and StrBuf

This is a stopgap until DST (#12938) lands.

Until DST lands, we cannot decompose &str into & and str, so we cannot
usefully take ToCStr arguments by reference (without forcing an
additional & around &str). So we are instead temporarily adding an
instance for &Path and StrBuf, so that we can take ToCStr as owned. When
DST lands, the &Path instance should be removed, the string instances
should be revisted, and arguments bound by ToCStr should be passed by
reference.

FIXMEs have been added accordingly.
Process::new etc should support non-utf8 commands/args

The existing APIs for spawning processes took strings for the command
and arguments, but the underlying system may not impose utf8 encoding,
so this is overly limiting.

The assumption we actually want to make is just that the command and
arguments are viewable as [u8] slices with no interior NULLs, i.e., as
CStrings. The ToCStr trait is a handy bound for types that meet this
requirement (such as &str and Path).

However, since the commands and arguments are often a mixture of
strings and paths, it would be inconvenient to take a slice with a
single T: ToCStr bound. So this patch revamps the process creation API
to instead use a builder-style interface, called `Command`, allowing
arguments to be added one at a time with differing ToCStr
implementations for each.

The initial cut of the builder API has some drawbacks that can be
addressed once issue #13851 (libstd as a facade) is closed. These are
detailed as FIXMEs.

Closes #11650.

[breaking-change]
Change dynamic_library::open_external to take ToCStr

`std::unstable::dynamic_library::open_external` currently takes a
`Path`, but because `Paths` produce normalized strings, this can
change the semantics of lookups in a given environment. This patch
generalizes the function to take a `ToCStr`-bounded type, which
includes both `Path`s and `str`s.

Closes #11650.
auto merge of #14174 : stepancheg/rust/once, r=alexcrichton

Submitting PR again, because I cannot reopen #13349, and github does not attach new patch to that PR. 

=======

Optimize `Once::doit`: perform optimistic check that initializtion is
already completed.  `load` is much cheaper than `fetch_add` at least
on x86_64.

Verified with this test:

```
static mut o: one::Once = one::ONCE_INIT;
unsafe {
    loop {
        let start = time::precise_time_ns();
        let iters = 50000000u64;
        for _ in range(0, iters) {
            o.doit(|| { println!("once!"); });
        }
        let end = time::precise_time_ns();
        let ps_per_iter = 1000 * (end - start) / iters;
        println!("{} ps per iter", ps_per_iter);

        // confuse the optimizer
        o.doit(|| { println!("once!"); });
    }
}
```

Test executed on Mac, Intel Core i7 2GHz. Result is:
* 20ns per iteration without patch
*  4ns per iteration with this patch applied

Once.doit could be even faster (800ps per iteration), if `doit` function
was split into a pair of `doit`/`doit_slow`, and `doit` marked as
`#[inline]` like this:

```
#[inline(always)]
pub fn doit(&self, f: ||) {
    if self.cnt.load(atomics::SeqCst) < 0 {
        return
    }

    self.doit_slow(f);
}

fn doit_slow(&self, f: ||) { ... }
```
auto merge of #14198 : thestinger/rust/valgrind, r=luqmana
auto merge of #13954 : aturon/rust/issue-11650, r=alexcrichton

## Process API

The existing APIs for spawning processes took strings for the command
and arguments, but the underlying system may not impose utf8 encoding,
so this is overly limiting.

The assumption we actually want to make is just that the command and
arguments are viewable as [u8] slices with no interior NULLs, i.e., as
CStrings. The ToCStr trait is a handy bound for types that meet this
requirement (such as &str and Path).

However, since the commands and arguments are often a mixture of
strings and paths, it would be inconvenient to take a slice with a
single T: ToCStr bound. So this patch revamps the process creation API
to instead use a builder-style interface, called `Command`, allowing
arguments to be added one at a time with differing ToCStr
implementations for each.

The initial cut of the builder API has some drawbacks that can be
addressed once issue #13851 (libstd as a facade) is closed. These are
detailed as FIXMEs.

## Dynamic library API

`std::unstable::dynamic_library::open_external` currently takes a
`Path`, but because `Paths` produce normalized strings, this can
change the semantics of lookups in a given environment. This patch
generalizes the function to take a `ToCStr`-bounded type, which
includes both `Path`s and `str`s.

## ToCStr API

Adds ToCStr impl for &Path and ~str. This is a stopgap until DST (#12938) lands.

Until DST lands, we cannot decompose &str into & and str, so we cannot
usefully take ToCStr arguments by reference (without forcing an
additional & around &str). So we are instead temporarily adding an
instance for &Path and ~str, so that we can take ToCStr as owned. When
DST lands, the &Path instance should be removed, the string instances
should be revisted, and arguments bound by ToCStr should be passed by
reference.

FIXMEs have been added accordingly. 

## Tickets closed

Closes #11650.
Closes #7928.
Include file paths when dumping the list of resolved crates via `debug!`.
Output debug info on how `#[phase] extern crate foo;` gets resolved.
Refactoring: Introduce distinct host and target rpath var setters.

`HOST_RPATH_VAR$(1)_T_$(2)_H_$(3)` and `TARGET_RPATH_VAR$(1)_T_$(2)_H_$(3)`
both match the format of the old `RPATH_VAR$(1)_T_$(2)_H_$(3)` (which
is still being set the same way that it was before, to one of either
HOST/TARGET depending on what stage we are building).  Namely, the format
is <XXX>_RPATH_VAR = "<LD_LIB_PATH_ENVVAR>=<COLON_SEP_PATH_ENTRIES>"

This should be a pure refactoring apart from the introduction of the
two new make environment variables.
Distinguish HOST_RPATH, TARGET_RPATH; cleanup mk files for stage1.

Two line summary: Distinguish HOST_RPATH and TARGET_RPATH; added
RPATH_LINK_SEARCH; skip tests broken in stage1; general cleanup.

What this commit does:

* Pass both of the (newly introduced) HOST and TARGET rpath setup vars
  to `maketest.py`

* Update `maketest.py` to no longer update the LD_LIBRARY_PATH itself
  Instead, it passes along the HOST and TARGET rpath setup vars in
  environment variables `HOST_RPATH_ENV` and `TARGET_RPATH_ENV`

* Cleanup: Distinguish in tools.mk between the command to run (`RUN`)
  and the file to generate to drive that command (`RUN_BINFILE`).  The
  main thing this enables is that `RUN` can now setup the
  `TARGET_RPATH_ENV` without having to dirty up the runner code in
  each of the `run-make` Makefiles.

* Pass stage to maketest.py; it in turn passes it (via an env var) to
  run-make tests.

  This allows the run-make tests to selectively change behavior
  (e.g. turn themselves off) to deal with incompatibilities with
  e.g. stage1.

* Factored out commands to delete dylib/rlib into REMOVE_DYLIBS/REMOVE_RLIBS.

  There were places where we were only calling `rm $(call DYLIB,foo)`
  even though we really needed to get rid of the whole glob (at least
  based on alex's findings on #13753 that removing the symlink does not
  suffice).

  Therefore rather than peppering the code with the awkward
  `rm $(TMPDIR)/$(call DYLIB_GLOB,foo)`, I instead introduced a common
  `REMOVE_DYLIBS` user function that expands into that when called.
  After I adding an analogous `REMOVE_RLIBS`, I changed all of the
  existing calls that rm dylibs or rlibs to use these routines
  instead.

  Note that the latter is not a true refactoring since I may have
  changed cases where it was our intent to only remove the sym-link.
  (But if that is the case, then we need to more deeply investigate
  alex's findings on #13753 where the system was still dynamically
  loading up the non-symlinked libraries that it finds on the load
  path.)

* Added RPATH_LINK_SEARCH command and use it on Linux.

  On some platforms, namely Linux, when you have libboot.so that has
  its internal rpath set (to e.g. $(ORIGIN)/path/to/HOSTDIR), the
  linker still complains when you do the link step and it does not
  know where to find libraries that libboot.so depends upon that live
  in HOSTDIR (think e.g. librustuv.so).

  As far as I can tell, the GNU linker will consult the
  LD_LIBRARY_PATH as part of the linking process to find such
  libraries.  But if you want to be more careful and not override
  LD_LIBRARY_PATH for the `gcc` invocation, then you need some other
  way to tell the linker where it can find the libraries that
  libboot.so needs.  The solution to this on Linux is the
  `-Wl,-rpath-link` command line option.

  However, this command line option does not exist on Mac OS X, (which
  appears to be figuring out how to resolve the libboot.dylib
  dependency by some other means, perhaps by consulting the rpath
  setting within libboot.dylib).

  So, in order to abstract over this distinction, I added the
  RPATH_LINK_SEARCH macro to the run-make infrastructure and added
  calls to it where necessary to get Linux working.  On architectures
  other than Linux, the macro expands to nothing.
Skip these tests at stage1.

Note: apparently you cannot quote operands to `ifeq` in Makefile.
Excuse regex from doing rustdoc tests atop stage1.

Hopefully final step to get `make check-stage1` working again.

The use of a special-case check for regex here is ugly but is
analogous other similar checks for regex such as the one that landed
in PR #13844.

The way this is written, the user will get a reminder that
doc-crate-regex is being skipped whenever their rules attempt to do
the crate documentation tests.  This is deliberate: I want people
running `make check-stage1` to be reminded about which cases are being
skipped.  (But if such echo noise is considered offensive, it can
obviously be removed.)
Fixing rustdoc stage1.

See #13983 and #14000.

Fix was originally authored by alexcrichton and then rebased a couple
times by pnkfelix, most recently atop PR 13954.