[lld-macho] Implement weak binding for branch relocations

Since there is no "weak lazy" lookup, function calls to weak symbols are
always non-lazily bound. We emit both regular non-lazy bindings as well
as weak bindings, in order that the weak bindings may overwrite the
non-lazy bindings if an appropriate symbol is found at runtime. However,
the bound addresses will still be written (non-lazily) into the
LazyPointerSection.

Reviewed By: #lld-macho, smeenai

Differential Revision: https://reviews.llvm.org/D86573