This commit cleans up allocator injection logic found in the compiler
around selecting the global allocator. It turns out that now that
jemalloc is gone the compiler never actually injects anything! This
means that basically everything around loading crates here and there can
be easily pruned.

This also removes the `exe_allocation_crate` option from custom target
specs as it's no longer used by the compiler anywhere.

This avoids trapping in the -Zsaturating-float-casts implementation.

Fuchsia already uses lld as the default linker, so there's no reason
to always invoke it through Clang, instead we can simply invoke lld
directly and pass the set of flags that matches Clang.

This commit deletes the injection of `-(` and `-)` options to the linker
for the musl targets. This actually causes problems today on nightly if
you execute:

    $ echo 'fn main() {}' >> foo.rs
    $ rustc --target x86_64-unknown-linux-musl -C panic=abort

you get a linker error about "cannot nest groups". This comes about
because rustc injects its own `--start-group` and `--end-group`
variables which clash with the outer `-(` and `-)` variables. It's not
entirely clear to me why this doesn't affect the musl target by default
(in `-C panic=unwind` mode).

The compiler's own injection of `--start-group` and `--end-group` should
solve the issues mentioned in the comment for injecting `-(` and `-)` as
well.

This commit adds opt-in support to the compiler to link to `jemalloc` in
the compiler. When activated the compiler will depend on `jemalloc-sys`,
instruct jemalloc to unprefix its symbols, and then link to it. The
feature is activated by default on Linux/OSX compilers for x86_64/i686
platforms, and it's not enabled anywhere else for now. We may be able to
opt-in other platforms in the future! Also note that the opt-in only
happens on CI, it's otherwise unconditionally turned off by default.

Closes #36963

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.

PR #51953 enabled the Atomic*.{load,store} API on MSP430. Unfortunately, the
LLVM backend doesn't currently support those atomic operations, so this commit
removes the API and leaves instructions on how and when to enable it in the
future.

The upcoming SIMD support in the wasm target is unique from the other
platforms where it's either unconditionally available or not available,
there's no halfway where a subsection of the program can use it but no
other parts of the program can use it. In this world it's valid for wasm
SIMD args to always be passed by value and there's no need to pass them
by reference.

This commit adds a new custom target specification option
`simd_types_indirect` which defaults to `true`, but the wasm backend
disables this and sets it to `false`.

Disable the PLT where possible to improve performance
for indirect calls into shared libraries.

This optimization is enabled by default where possible.

- Add the `NonLazyBind` attribute to `rustllvm`:
  This attribute informs LLVM to skip PLT calls in codegen.

- Disable PLT unconditionally:
  Apply the `NonLazyBind` attribute on every function.

- Only enable no-plt when full relro is enabled:
  Ensures we only enable it when we have linker support.

- Add `-Z plt` as a compiler option

MSVC ignores these keywords for C/C++ and uses the standard system
calling convention.  Rust should do so as well.

Fixes #54569.

The no_default_libraries was introduced in #28578 because the
NetBSD-based rumprun needed to disable the link flag.
This moves the definition to be used by all NetBSD linker flavors to
close #49627.

A different solution would be adding -lc but as there is no platform
with explicit -lc, this approach is used.

This commit adds a few changes for atomic instructions on the
`wasm32-unknown-unknown` target. Atomic instructions are not yet stable in
WebAssembly itself but there are multiple implementations and LLVM has support
for the proposed instruction set, so let's work on exposing it!

Here there are a few inclusions:

* The `atomics` feature was whitelisted for LLVM, allowing code in Rust to
  enable/disable/gate on this.

* The `singlethread` option is turned off for wasm when the `atomics` feature is
  enabled. This means that by default wasm won't be lowering with atomics, but
  when atomics are enabled globally we'll turn off single-threaded mode to
  actually codegen atomics. This probably isn't what we'll want in the long term
  but for now it should work.

* Finally the maximum atomic width is increased to 64 to reflect the current
  wasm spec.

by removing the redundant +v7 feature

Similar to `armebv7r-none-eabihf`, but for Little-endian MCUs.

As example TI RM4x/RM5x are Little-endian Cortex-R4F/R5F MCUs.

CI/Dockerfile is intentionally in the disabled folder.

to rust-lld so users won't need an external linker to build programs

this field defaults to the LD / GNU flavor

This commit adds the necessary definitions for target specs and such as well as
the necessary support in libstd to compile basic `aarch64-pc-windows-msvc`
binaries. The target is not currently built on CI, but it can be built locally
with:

    ./configure --target=aarch64-pc-windows-msvc --set rust.lld
    ./x.py build src/libstd --target aarch64-pc-windows-msvc

Currently this fails to build `libtest` due to a linker bug (seemingly in LLD?)
which hasn't been investigate yet. Otherwise though with libstd you can build a
hello world program (linked with LLD). I've not tried to execute it yet, but it
at least links!

Full support for this target is still a long road ahead, but this is hopefully a
good stepping stone to get started.

Points of note about this target are:

* Currently defaults to `panic=abort` as support is still landing in LLVM for
  SEH on AArch64.
* Currently defaults to LLD as a linker as I was able to get farther with it
  than I was with `link.exe`

A generic AArch64 target that can be used for writing bare-metal code
for 64-bit ARM architectures.

Same as the other embedded targets, see:
https://github.com/rust-lang/rust/pull/49728

This is a temporary workaround for #44993.