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

[Louis Dionne via cfe-commits]

ldionne wrote:

Since we support only the latest released GCC in the runtimes, so I don't think it makes sense to merge this. Newer GCCs don't seem to have this issue.

Closing, please reopen if you'd like to pursue this despite the above information and we can chat.

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