[lld-macho] Move adding bindings for stub targets out of Writer (NFC)

We now re-use the existing needsBinding() helper to determine if a
branch has to go through a stub. The logic for determining which type of
binding is needed is moved inside StubsSection::addEntry().

This is an NFC refactor that simplifies my diff that adds support for
chained fixups.

Differential Revision: https://reviews.llvm.org/D132476