[lld-macho] Add 32-bit compact unwind support

This could probably have been part of D99633, but I split it up to make
things a bit more reviewable. I also fixed some bugs in the implementation that
were masked through integer underflows when operating in 64-bit mode.

Reviewed By: #lld-macho, gkm

Differential Revision: https://reviews.llvm.org/D99823