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

[Mosè Giordano via cfe-commits]

giordano wrote:

Based on https://github.com/JuliaPackaging/Yggdrasil/pull/9861/files#diff-812a6db0df5a0d42010930d41a801e33e89c6064172903e5aacb88a7c704def2R7 this came up with gcc 10

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