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

[Jon Roelofs via cfe-commits]

jroelofs wrote:

mind adding a test? you could probably piggyback on `bad_unwind_info.pass.cpp`'s second `assert`.

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