[compiler-rt] [libcxxabi] [libunwind] [PAC][libunwind] Fix gcc build of libunwind and compiler-rt (PR #164535)

[via llvm-commits]

dyung wrote:

> > Will this cover the use in `compiler-rt/lib/builtins/gcc_personality_v0.c`?
> 
> adding guards to that as well. There are enough has_feature guards there that I assumed it was a clang only project.

Thanks! We definitely do build internally with GCC as well as clang, and I'm pretty sure others do build with gcc as well (although compiler-rt may not be built as often as llvm or clang).

https://github.com/llvm/llvm-project/pull/164535