[libunwind] [libunwind] Add GCS support for AArch64 (PR #99335)

[John Brawn via cfe-commits]

john-brawn-arm wrote:

> This seems to add a new feature, is it really relevant for a backport?

This is the last piece of work to enable GCS in the LLVM toolchain (clang and lld support went in a while ago), and I was hoping to get everything in to LLVM 19. It would be rather annoying to have this last part slip to LLVM 20.

The way GCS works is that the `-mbranch-protection=standard` command-line option marks generated objects with the GCS property, and lld then sets the GCS property on the output if all inputs have it, which then indicates to the operating system to enable GCS on that process. Without GCS support in libunwind that means it won't have the GCS property so anything that links with libunwind will have GCS disabled.

Though having said all this, I marked this for LLVM 19 as my reading of https://discourse.llvm.org/t/update-on-llvm-19-x-releases/80511 was that new features are still ok before RC2, but re-reading it it's a bit ambiguous: it says "New features will have to wait until LLVM 20 at this point" and my reading of that was that "this point" means RC2, so new features before RC2 is fine, but if "this point" meant "right now" that means new features aren't OK. If that's the case the it's reasonable for this to not go in RC2.

https://github.com/llvm/llvm-project/pull/99335