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

[Jon Roelofs via cfe-commits]

jroelofs wrote:

The other libunwind has a different license. Copying code from that one into this one is not something we should do: https://github.com/llvm/llvm-project/pull/160887#discussion_r2410120315

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