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

[Gleb Popov via cfe-commits]

arrowd wrote:

> I deliberately didn't talk about that in this part of my message because it's not relevant; adding code does not affect whether the rest of the code is a license violation.

Yes, but it is also a sort of evidence that I didn't just blindly copy the nongnu code.

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