Make Vec.truncate() resilient against failure in Drop

If a vector element fails in Drop, Vec needs to make sure it doesn't try
to re-drop any elements it already dropped.
Register new snapshots
use sched_yield on linux and freebsd

pthread_yield is non-standard, while sched_yield is POSIX

The Linux documentation recommends using the standard function. This is
the only feature we're currently using that's present in glibc but not
in musl.
Add a crate for missing stubs from libcore

The core library in theory has 0 dependencies, but in practice it has some in
order for it to be efficient. These dependencies are in the form of the basic
memory operations provided by libc traditionally, such as memset, memcmp, etc.
These functions are trivial to implement and themselves have 0 dependencies.

This commit adds a new crate, librlibc, which will serve the purpose of
providing these dependencies. The crate is never linked to by default, but is
available to be linked to by downstream consumers. Normally these functions are
provided by the system libc, but in other freestanding contexts a libc may not
be available. In these cases, librlibc will suffice for enabling execution with
libcore.

cc #10116
Render not_found with an absolute path to the rust stylesheet
update valgrind headers
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: ||) { ... }
```
Print 'rustc' and 'rustdoc' as the command name for --version

Instead of just blindly printing arg[0], use a constant string.
Partial fix for #13582
syntax::visit: pub `walk_explicit_self` so impls can call it as defaults do.

drive-by: added some doc.
Teach SVH computation to ignore more implementation artifacts.

In particular, this version of strict version hash (SVH) works much
like the deriving(Hash)-based implementation did, except that uses a
content-based hash that filters rustc implementation artifacts and
surface syntax artifacts.

Fix #14132.
Some basic acceptance tests for better SVH.
A test case for a bug I found in the new SVH while reviewing it.

Namely: non-pub `use` declarations *are* significant to the SVH
computation, since they can change which traits are part of the method
resolution step, and thus affect which methods get called from the
(potentially inlined) code.
Added tests checking that changes in type sig are recognized in SVH.

(Only after adding the tests did I realize that this is not really a
special case at the AST level; as far as the visitor is concerned,
`int` and `i32` and `i64` are just idents.)
Test fixes from rollup

Closes #14210 (Make Vec.truncate() resilient against failure in Drop)
Closes #14206 (Register new snapshots)
Closes #14205 (use sched_yield on linux and freebsd)
Closes #14204 (Add a crate for missing stubs from libcore)
Closes #14201 (Render not_found with an absolute path to the rust stylesheet)
Closes #14198 (update valgrind headers)
Closes #14174 (Optimize common path of Once::doit)
Closes #14162 (Print 'rustc' and 'rustdoc' as the command name for --version)
Closes #14145 (Better strict version hash (SVH) computation)
auto merge of #14214 : alexcrichton/rust/rollup, r=alexcrichton