[lld-macho] Implement weak bindings for GOT/TLV

Previously, we were only emitting regular bindings to weak
dynamic symbols; this diff adds support for the weak bindings too, which
can overwrite the regular bindings at runtime. We also treat weak
defined global symbols similarly -- since they can also be interposed at
runtime, they need to be treated as potentially dynamic symbols.

Note that weak bindings differ from regular bindings in that they do not
specify the dylib to do the lookup in (i.e. weak symbol lookup happens
in a flat namespace.)

Differential Revision: https://reviews.llvm.org/D86572