[compiler-rt] [libcxx] [libcxxabi] [libunwind] [runtimes][PAC] Harden unwinding when possible (PR #143230)

[Oliver Hunt via cfe-commits]

ojhunt wrote:

Will fix momentarily. I guess gcc does not have `__has_feature`, so I'll look at how libunwind usually does this. We may just gate on the AARCH_PTRAUTH flag instead.

https://github.com/llvm/llvm-project/pull/143230