[lld-macho] Properly test subtractor relocations & fix their attributes

`llvm-mc` doesn't generate any relocations for subtractions
between local symbols -- they must be global -- so the previous test
wasn't actually testing any relocation logic. I've fixed that and
extended the test to cover r_length=3 relocations as well as both x86_64
and arm64.

Reviewed By: #lld-macho, smeenai

Differential Revision: https://reviews.llvm.org/D97057