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

[Jon Roelofs via cfe-commits]

jroelofs wrote:

I closed it out of an abundance of caution because you wrote:

> I took message strings from nongnu libunwind: [https://github.com/libunwind/libunwind/blob/bfc0d618f72ba4b725c3735188e0a6d8c710411c/src/mi/strerror.c#L3](https://github.com/libunwind/libunwind/blob/bfc0d618f72ba4b725c3735188e0a6d8c710411c/src/mi/strerror.c#L31)

I want to make it clear that we CANNOT copy code out of another implementation with another license.

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