rustc: Disable rpath settings by default

This commit disables rustc's emission of rpath attributes into dynamic libraries
and executables by default. The functionality is still preserved, but it must
now be manually enabled via a `-C rpath` flag.

This involved a few changes to the local build system:

* --disable-rpath is now the default configure option
* Makefiles now prefer our own LD_LIBRARY_PATH over the user's LD_LIBRARY_PATH
  in order to support building rust with rust already installed.
* The compiletest program was taught to correctly pass through the aux dir as a
  component of LD_LIBRARY_PATH in more situations.

The major impact of this change is that neither rustdoc nor rustc will work
out-of-the-box in all situations because they are dynamically linked. It must be
arranged to ensure that the libraries of a rust installation are part of the
LD_LIBRARY_PATH. The default installation paths for all platforms ensure this,
but if an installation is in a nonstandard location, then configuration may be
necessary.

Additionally, for all developers of rustc, it will no longer be possible to run
$target/stageN/bin/rustc out-of-the-box. The old behavior can be regained
through the `--enable-rpath` option to the configure script.

This change brings linux/mac installations in line with windows installations
where rpath is not possible.

Closes #11747
[breaking-change]
librustc: Apply null pointer optimization to slices, closures and trait
objects.
librustc: Remove match arm since we don't allow enum to float casts.
librustc: Don't create &[T] slices with NULL as the ptr.
Add tests for null pointer opt.
librustc: Accept type aliases for structures in structure literals and
structure patterns.

Closes #4508.
Update RELEASES.txt

Fix Typos
libcore: Fix Items iterator for zero sized types.
Fix #15453
Improved example in the Guide

Fixes https://github.com/rust-lang/rust/issues/15452
Optimize String::push_byte()

```
test new_push_byte ... bench:      6985 ns/iter (+/- 487) = 17 MB/s
test old_push_byte ... bench:     19335 ns/iter (+/- 1368) = 6 MB/s
```

```rust
extern crate test;
use test::Bencher;

static TEXT: &'static str = "\
    Unicode est un standard informatique qui permet des échanges \
    de textes dans différentes langues, à un niveau mondial.";

#[bench]
fn old_push_byte(bencher: &mut Bencher) {
    bencher.bytes = TEXT.len() as u64;
    bencher.iter(|| {
        let mut new = String::new();
        for b in TEXT.bytes() {
            unsafe { new.as_mut_vec().push_all([b]) }
        }
    })
}

#[bench]
fn new_push_byte(bencher: &mut Bencher) {
    bencher.bytes = TEXT.len() as u64;
    bencher.iter(|| {
        let mut new = String::new();
        for b in TEXT.bytes() {
            unsafe { new.as_mut_vec().push(b) }
        }
    })
}
```
doc: --version no longer displays the host triple
auto merge of #15447 : patrickyevsukov/rust/patch-1, r=alexcrichton

Fix Typos
auto merge of #15454 : jakub-/rust/15453, r=huonw

I forget we now have byte string literals.
auto merge of #15456 : aochagavia/rust/guide, r=alexcrichton

Fixes https://github.com/rust-lang/rust/issues/15452
mk: Fix bootstrapping the nightly builds

The stage0 compiler for a non-CFG_BUILD architecture needs to have the new
`-C extra-filename` argument passed.
auto merge of #15465 : SimonSapin/rust/patch-4, r=alexcrichton

`Vec::push_all` with a length 1 slice seems to have significant overhead compared to `Vec::push`.

```
test new_push_byte ... bench:      6985 ns/iter (+/- 487) = 17 MB/s
test old_push_byte ... bench:     19335 ns/iter (+/- 1368) = 6 MB/s
```

```rust
extern crate test;
use test::Bencher;

static TEXT: &'static str = "\
    Unicode est un standard informatique qui permet des échanges \
    de textes dans différentes langues, à un niveau mondial.";

#[bench]
fn old_push_byte(bencher: &mut Bencher) {
    bencher.bytes = TEXT.len() as u64;
    bencher.iter(|| {
        let mut new = String::new();
        for b in TEXT.bytes() {
            unsafe { new.as_mut_vec().push_all([b]) }
        }
    })
}

#[bench]
fn new_push_byte(bencher: &mut Bencher) {
    bencher.bytes = TEXT.len() as u64;
    bencher.iter(|| {
        let mut new = String::new();
        for b in TEXT.bytes() {
            unsafe { new.as_mut_vec().push(b) }
        }
    })
}
```
auto merge of #15467 : brson/rust/guideversion, r=alexcrichton
Add a test case for #12187, which appears to have been fixed

Closes #12187.
Remove remainders from when booleans were i8