Add support for MIPS to LLVM's libunwind.

[jhb]

Summary

This is originally based on a patch from David Chisnall for soft-float
N64 but has since been updated to support O32, N32, and hard-float ABIs.
The soft-float O32, N32, and N64 support has been committed upstream.
The hard-float changes are still in review upstream.

Enable LLVM_LIBUNWIND on mips when building with a suitable (C+11-capable)
toolchain. This has been tested with external GCC for all ABIs and
O32 and N64 with clang.

Test Plan

• Upstream has run libunwind's tests on the committed changes. I wrote a simple C++ program that invokes _Unwind_Backtrace and throws a few exceptions and verified that worked when using this unwinder. Modern GDB (which depends on working C++ exceptions) also works fine using this unwinder.

• Given that the hard-float change is still in review, we might want to wait for that to finish (and that was my original plan). However, I also have some desire to get this out of my tree where it's been lingering for months.

[emaste]

How much is currently upstream?

[jhb]

All but the hard float changes are upstream. The only patch in review still is https://reviews.llvm.org/D41968