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

[Alexander Richardson via cfe-commits]

arichardson wrote:

I'm not convinced that reusing the error strings counts as some form of copyright violation especially since the same value may in theory be required for API compatibility and therefore should be exempt. But I'm not a copyright lawyer. 

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