[libunwind] libunwind: Implement the unw_strerror function for better nongnu libunwind compatibility (PR #160887)

[Alexander Richardson via cfe-commits]

https://github.com/arichardson approved this pull request.

LGTM with the error string changed. All these strings are already part of libunwind.h, so nothing changes there.

https://github.com/llvm/llvm-project/pull/160887