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

[Louis Dionne via cfe-commits]

ldionne wrote:

Ah, I didn't realize this was only required for much older compilers. In that case, indeed we don't support GCC 8 so we probably don't want to merge this.

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