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

[Oliver Hunt via llvm-commits]

ojhunt wrote:

I think polluting space with `__has_feature` is the wrong approach, will gate on `__PTRAUTH__ ||  __arm64e__` if you can verify that `__PTRAUTH__` is defined for you folk when building the .S files

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