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

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Zentrik wrote:

Based on https://github.com/WAVM/WAVM/pull/266#issue-583580780 and https://en.cppreference.com/w/cpp/types/integer#Notes this might have been fixed in glibc 2.18. 

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