Enable xray support for Mac

[quininer]

#102921

Upstream has supported Mac for a while, let's enable it.

I've tested it on M4 and it generates nop sled correctly.

• https://maskray.me/blog/2023-06-18-port-llvm-xray-to-apple-systems
• https://github.com/llvm/llvm-project/blob/llvmorg-20.1.4/clang/lib/Driver/XRayArgs.cpp#L31

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These commits modify compiler targets.
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[jieyouxu]

Question: do we have any smoke test for xray on x86_64-apple-darwin and aarch64-apple-darwin? As in:

• A smoke test that have some ability to observe xray being enabled or not, for aarch64-apple-darwin and x86_64-apple-darwin targets.
• A test check that the flag -Z instrument-xray is still unstable, but is no longer rejected on x86_64-apple-darwin and aarch64-apple-darwin respectively.

[quininer]

I think both of these tests don't exist at the moment, I can add tests for this but there are two problems

1. It seems that only llir is tested in tests/codegen and no asm, which cannot be used to check whether xray is enabled, because xray affects llvm output rather than input. i need some guidance how to write tests for asm output.
2. Add a test specifically for instrument-xray + x86_64-apple-darwin/aarch64-apple-darwin combination seems a bit strange to me, since this is not a feature we implement, but just an upstream feature. do we need to add such a test for each supported target?

[jieyouxu]

Since this affects the Tier 1 {x86_64,aarch64}-apple-darwin targets (albeit -Z instrument-xray is still unstable), asking Apple experts for second opinion as I neither have access to apple envs nor am I aware of how well LLVM xray currently works on those targets.

@rustbot ping apple

[rustbot]

Hey Apple notification group! This issue or PR could use some Apple-specific
guidance. Could one of you weigh in? Thanks <3

(In case it's useful, here are some instructions for tackling these sorts of
issues).

cc @BlackHoleFox @hkratz @inflation @madsmtm @nvzqz @shepmaster @thomcc

[jieyouxu]

@rustbot author (test nits)

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.