[lld-macho] Flesh out STABS implementation

This addresses a lot of the comments in {D89257}. Ideally it'd have been
done in the same diff, but the commits in between make that difficult.

This diff implements:
* N_GSYM and N_STSYM, the STABS for global and static symbols
* Has the STABS reflect the section IDs of their referent symbols
* Ensures we don't fail when encountering absolute symbols or files with
  no debug info
* Sorts STABS symbols by file to minimize the number of N_OSO entries

Reviewed By: clayborg

Differential Revision: https://reviews.llvm.org/D92366