[libunwind] [libunwind] Make sure `__STDC_FORMAT_MACROS` is defined to ensure `PRId64` is available (PR #117491)

[Fangrui Song via cfe-commits]

MaskRay wrote:

What old GCC versions need this? If it's just GCC 8, it's possible that we will drop GCC buildability soon. I think for runtime libraries we could pose a stricter requirement.

https://github.com/llvm/llvm-project/pull/117491