[libunwind] [libunwind][AArch64] Protect PC within libunwind's context. (PR #113368)

[Oliver Hunt via cfe-commits]

ojhunt wrote:

I've finally got our downstream hardening to the point where it should be sane: https://github.com/llvm/llvm-project/pull/143230

https://github.com/llvm/llvm-project/pull/113368