`target.target_endian` -> `target.endian`
`target.target_c_int_width` -> `target.c_int_width`
`target.target_os` -> `target.os`
`target.target_env` -> `target.env`
`target.target_vendor` -> `target.vendor`
`target.target_family` -> `target.os_family`
`target.target_mcount` -> `target.mcount`

2. change armv7_wrs_vxworks to armv7_wrs_vxworks_eabihf.
3. use wr-** instead of vx-**
4. set PIE to false
5. code cleanup

r? @alexcrichton

This commit removes all jemalloc related submodules, configuration, etc,
from the bootstrap, from the standard library, and from the compiler.
This will be followed up with a change to use jemalloc specifically as
part of rustc on blessed platforms.

Signed-off-by: NJohannes Löthberg <johannes@kyriasis.com>

This commit adds support for full RELRO, and enables it for the
platforms I know have support for it.

Full RELRO makes the PLT+GOT data read-only on startup, preventing it
from being overwritten.

http://tk-blog.blogspot.com/2009/02/relro-not-so-well-known-memory.html

Fixes rust-lang/rust#29877.
Signed-off-by: NJohannes Löthberg <johannes@kyriasis.com>

This patch adds a `-Z linker-flavor` flag to rustc which can be used to invoke
the linker using a different interface.

For example, by default rustc assumes that all the Linux targets will be linked
using GCC. This makes it impossible to use LLD as a linker using just `-C
linker=ld.lld` because that will invoke LLD with invalid command line
arguments. (e.g. rustc will pass -Wl,--gc-sections to LLD but LLD doesn't
understand that; --gc-sections would be the right argument)

With this patch one can pass `-Z linker-flavor=ld` to rustc to invoke the linker
using a LD-like interface. This way, `rustc -C linker=ld.lld -Z
linker-flavor=ld` will invoke LLD with the right arguments.

`-Z linker-flavor` accepts 4 different arguments: `em` (emcc), `ld`,
`gcc`, `msvc` (link.exe). `em`, `gnu` and `msvc` cover all the existing linker
interfaces. `ld` is a new flavor for interfacing GNU's ld and LLD.

This patch also changes target specifications. `linker-flavor` is now a
mandatory field that specifies the *default* linker flavor that the target will
use. This change also makes the linker interface *explicit*; before, it used to
be derived from other fields like linker-is-gnu, is-like-msvc,
is-like-emscripten, etc.

Another change to target specifications is that the fields `pre-link-args`,
`post-link-args` and `late-link-args` now expect a map from flavor to linker
arguments.

``` diff
-    "pre-link-args": ["-Wl,--as-needed", "-Wl,-z,-noexecstack"],
+    "pre-link-args": {
+        "gcc": ["-Wl,--as-needed", "-Wl,-z,-noexecstack"],
+        "ld": ["--as-needed", "-z,-noexecstack"],
+    },
```

[breaking-change]  for users of custom targets specifications

The compiler currently vendors its own version of "llvm-ar" (not literally the
binary but rather the library support) and uses it for all major targets by
default (e.g. everything defined in `src/librustc_back/target`). All custom
target specs, however, still search for an `ar` tool by default. This commit
changes this default behavior to using the internally bundled llvm-ar with the
GNU format.

Currently all targets use the GNU format except for OSX which uses the BSD
format (surely makes sense, right?), and custom targets can change the format
via the `archive-format` key in custom target specs.

I suspect that we can outright remove support for invoking an external `ar`
utility, but I figure for now there may be some crazy target relying on that so
we should leave support in for now.

Currently the standard library has some pretty complicated logic to detect
whether #[thread_local] should be used or whether it's supported. This is also
unfortunately not quite true for OSX where not all versions support
the #[thread_local] attribute (only 10.7+ does). Compiling code for OSX 10.6 is
typically requested via the MACOSX_DEPLOYMENT_TARGET environment variable (e.g.
the linker recognizes this), but the standard library unfortunately does not
respect this.

This commit updates the compiler to add a `target_thread_local` cfg annotation
if the platform being targeted supports the `#[thread_local]` attribute. This is
feature gated for now, and it is only true on non-aarch64 Linux and 10.7+ OSX
(e.g. what the module already does today). Logic has also been added to parse
the deployment target environment variable.

This commit updates the compiler to not attempt to use jemalloc for platforms
where jemalloc is never enabled. Currently the compiler attempts to link in
jemalloc based on whether `--disable-jemalloc` was specified at build time for
the compiler itself, but this is only the right decision for the host target,
not for other targets.

This still leaves a hole open where a set of target libraries are downloaded
which were built with `--disable-jemalloc` and the compiler is unaware of that,
but this is a pretty rare case so it can always be fixed later.

This commit is an implementation of [RFC 1183][rfc] which allows swapping out
the default allocator on nightly Rust. No new stable surface area should be
added as a part of this commit.

[rfc]: https://github.com/rust-lang/rfcs/pull/1183

Two new attributes have been added to the compiler:

* `#![needs_allocator]` - this is used by liballoc (and likely only liballoc) to
  indicate that it requires an allocator crate to be in scope.
* `#![allocator]` - this is a indicator that the crate is an allocator which can
  satisfy the `needs_allocator` attribute above.

The ABI of the allocator crate is defined to be a set of symbols that implement
the standard Rust allocation/deallocation functions. The symbols are not
currently checked for exhaustiveness or typechecked. There are also a number of
restrictions on these crates:

* An allocator crate cannot transitively depend on a crate that is flagged as
  needing an allocator (e.g. allocator crates can't depend on liballoc).
* There can only be one explicitly linked allocator in a final image.
* If no allocator is explicitly requested one will be injected on behalf of the
  compiler. Binaries and Rust dylibs will use jemalloc by default where
  available and staticlibs/other dylibs will use the system allocator by
  default.

Two allocators are provided by the distribution by default, `alloc_system` and
`alloc_jemalloc` which operate as advertised.

Closes #27389

This commit removes all morestack support from the compiler which entails:

* Segmented stacks are no longer emitted in codegen.
* We no longer build or distribute libmorestack.a
* The `stack_exhausted` lang item is no longer required

The only current use of the segmented stack support in LLVM is to detect stack
overflow. This is no longer really required, however, because we already have
guard pages for all threads and registered signal handlers watching for a
segfault on those pages (to print out a stack overflow message). Additionally,
major platforms (aka Windows) already don't use morestack.

This means that Rust is by default less likely to catch stack overflows because
if a function takes up more than one page of stack space it won't hit the guard
page. This is what the purpose of morestack was (to catch this case), but it's
better served with stack probes which have more cross platform support and no
runtime support necessary. Until LLVM supports this for all platform it looks
like morestack isn't really buying us much.

cc #16012 (still need stack probes)
Closes #26458 (a drive-by fix to help diagnostics on stack overflow)

We have previously always relied upon an external tool, `ar`, to modify archives
that the compiler produces (staticlibs, rlibs, etc). This approach, however, has
a number of downsides:

* Spawning a process is relatively expensive for small compilations
* Encoding arguments across process boundaries often incurs unnecessary overhead
  or lossiness. For example `ar` has a tough time dealing with files that have
  the same name in archives, and the compiler copies many files around to ensure
  they can be passed to `ar` in a reasonable fashion.
* Most `ar` programs found do **not** have the ability to target arbitrary
  platforms, so this is an extra tool which needs to be found/specified when
  cross compiling.

The LLVM project has had a tool called `llvm-ar` for quite some time now, but it
wasn't available in the standard LLVM libraries (it was just a standalone
program). Recently, however, in LLVM 3.7, this functionality has been moved to a
library and is now accessible by consumers of LLVM via the `writeArchive`
function.

This commit migrates our archive bindings to no longer invoke `ar` by default
but instead make a library call to LLVM to do various operations. This solves
all of the downsides listed above:

* Archive management is now much faster, for example creating a "hello world"
  staticlib is now 6x faster (50ms => 8ms). Linking dynamic libraries also
  recently started requiring modification of rlibs, and linking a hello world
  dynamic library is now 2x faster.
* The compiler is now one step closer to "hassle free" cross compilation because
  no external tool is needed for managing archives, LLVM does the right thing!

This commit does not remove support for calling a system `ar` utility currently.
We will continue to maintain compatibility with LLVM 3.5 and 3.6 looking forward
(so the system LLVM can be used wherever possible), and in these cases we must
shell out to a system utility. All nightly builds of Rust, however, will stop
needing a system `ar`.

This value is the default anyway

This commit adds support for x86_64-unknown-linux-musl as a target of the
compiler. There's some comments in the commit about some of the more flavorful
flags passed to the linker as it's not quite as trivial as the normal specs.

Removes all target-specific knowledge from rustc. Some targets have changed
during this, but none of these should be very visible outside of
cross-compilation. The changes make our targets more consistent.

iX86-unknown-linux-gnu is now only available as i686-unknown-linux-gnu. We
used to accept any value of X greater than 1. i686 was released in 1995, and
should encompass the bare minimum of what Rust supports on x86 CPUs.

The only two windows targets are now i686-pc-windows-gnu and
x86_64-pc-windows-gnu.

The iOS target has been renamed from arm-apple-ios to arm-apple-darwin.

A complete list of the targets we accept now:

arm-apple-darwin
arm-linux-androideabi
arm-unknown-linux-gnueabi
arm-unknown-linux-gnueabihf

i686-apple-darwin
i686-pc-windows-gnu
i686-unknown-freebsd
i686-unknown-linux-gnu

mips-unknown-linux-gnu
mipsel-unknown-linux-gnu

x86_64-apple-darwin
x86_64-unknown-freebsd
x86_64-unknown-linux-gnu
x86_64-pc-windows-gnu

Closes #16093

[breaking-change]