[lld-macho] Support subtractor relocations that reference sections

The minuend (but not the subtrahend) can reference a section.

Note that we do not yet properly validate that the subtrahend isn't
referencing a section; I've filed PR50034 to track that.

I've also extended the reloc-subtractor.s test to reorder symbols, to
make sure that the addends are being associated with the minuend (and not
the subtrahend) relocation.

Fixes PR49999.

Reviewed By: #lld-macho, thakis

Differential Revision: https://reviews.llvm.org/D100804