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

[Jessica Clarke via cfe-commits]

jrtc27 wrote:

> > the variable names and formatting are different, it uses early return rather than variable assignment, and this even has the default case after the end of the switch rather than inside it
> 
> It also has a LLVM-specific `UNW_ECROSSRASIGNING` case, which is absent in nongnu libunwind.

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.

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