Stabilise `aarch64_target_feature`

[adamgemmell]

This issue constitutes the stabilisation report as per the rustc dev guide. Tracking issue: #44839

This came about as a result of the discussion on stabilising the aarch64 runtime feature detection macro: #86941

Stabilisation PR: #90621

Summary

Along with std::is_aarch64_feature_detected!, we'd like to stabilise aarch64_target_feature. This allows using the target_feature attribute on aarch64 platforms like such:

#[target_feature(enable = "neon")]
unsafe fn asimd_code() {
    ...

This allows for unconditional code generation - the above function will be compiled as if assumed to only run on hosts that support the neon feature.

Note that this differs from compile-time feature detection (via cfg!) and run-time feature detection (via the std::is_aarch64_feature_detected! macro).

Test cases

There are a variety of x86_64 tests in place testing the behaviour of the target_feature attribute. Since this proposal does not change the behaviour of target_feature, only stabilises some of its inputs, I do not believe there's any pressing need to update these tests.

Documentation

I've updated the Rust reference: rust-lang/reference#1102

Unresolved questions

It's possible in the future that PAuth will have reason to be split into two separate features (paca and pacg in Linux's AT_HWCAP. If this happens the plan would be to add two new target_feature options and, if necessary, keep PAuth as the combination of the two. It doesn't make sense to pre-emptively separate them before they're separate in LLVM, nor keep PAuth unstable for an undetermined amount of time.