[release/7.0-staging] define UNW_AARCH64 aliases conditionally

[VSadov]

backport of: #84591

Customer Impact

Fixes a build break with the new macOS SDK.

---

The libunwind.h that comes with the new MacOS SDK now has the enum for things like UNW_AARCH64_X19, so the workaround for not having those constants now causes build failures due to duplicate definition.

runtime/src/coreclr/pal/src/exception/seh-unwind.cpp

Line 58 in 4ece8f0

// MacOS uses ARM64 instead of AARCH64 to describe these registers

Without taking build dependency on the newest SDK, we need to apply the workaround conditionally.
Since this is an enum (not a define), we can't do simple #ifndef and need to do a configure test.

Testing

Risk

[janvorli]

LGTM, thank you!

[jeffschwMSFT]

approved. we will take for consideration in 7.0.x

[VSadov]

Curious - what is needed for check-service-labels / check-labels (pull_request) to pass? I assumed the failure will have some info, but it appears very generic - Error: Process completed with exit code 1.

Is it just something that will pass when this is servicing approved ?

[janvorli]

@VSadov just a wild guess - maybe the missing milestone is causing that?

[VSadov]

Thanks!