[release/9.0] [NativeAOT] Introduce pointer-based CompareExchange intrinsic and use operating with syncblock bits.

[github-actions]

Backport of #106703 to release/9.0

/cc @VSadov

Customer Impact

• Customer reported
• Found internally

When a NativeAOT app is compiled for Debug, use of monitor locks (as in lock(obj){...} statement), could in rare cases lock a wrong object or cause a crash in GC.

This was causing intermittent build failures in CI, about once a day, when the bug made it into toolset and ILC itself was impacted.

Regression

• Yes
• No

The root cause is pre-9.0, but was not exposed as a bug due to several mitigating factors.
Making CompareExchange intrinsic self-referential in #92974 (Implement Interlocked for small types) indirectly exposed the bug.

Since this is a stress bug with relatively rare occurrence, it took a while for the issue to propagate to the toolset compiler and then present itself in a form of CI build failures due to crashing ILC

Testing

Verified locally by examining the codegen for both x64 and arm64, and by running a directed repro in a loop 300 times.
(with directed repro without a fix a crash happens after 5-10 iterations)

There is no way to add a regression test for this right now. We mostly rely on CoreCLR for GC-stress, but this scenario is one of a few cases specific to NativeAOT and not reachable in CoreCLR stress tests.
We will be discussing how to improve our GC-stress situation with NativeAOT.

Risk

Low. The actual change is fairly small. We rely on existing JIT code that expands CompareExchange intrinsic for ref int, just introduced an internal entry point that takes int* as location - so that unsafe GC byref to an object syncblock will have no way to show up.
Regardless if CompareExchange intrinsic is expanded or not, the new entry point has no byrefs in it. Emitted code is the same as before, but without GC tracking of the location argument.

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[jeffschwMSFT]

approved. we can merge when ready

[VSadov]

Thanks!!