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

[Oliver Hunt via llvm-commits]

ojhunt wrote:

Among other things I realized I didn't actually intended to include changes to libcxxabi in the first place (I started doing the work before finding other uses of __has_feature in it, so am hoping this will be fine).

For the .c and .S I've just adopted the noop `#define __has_feature`

A bit of search shows gcc13.3 was released in 2024 so supporting it seems reasonable.

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