[lld-macho] Support dynamic linking of thread-locals

References to symbols in dylibs work very similarly regardless of
whether the symbol is a TLV. The main difference is that we have a
separate `__thread_ptrs` section that acts as the GOT for these
thread-locals.

We can identify thread-locals in dylibs by a flag in their export trie
entries, and we cross-check it with the relocations that refer to them
to ensure that we are not using a GOT relocation to reference a
thread-local (or vice versa).

Reviewed By: #lld-macho, smeenai

Differential Revision: https://reviews.llvm.org/D85081