[lld-macho] Force higher alignment for __thread_vars

`__thread_vars` contains pointers to `__tlv_bootstrap`, which are fixed
up by dyld; however the section's alignment is not specified. This means
that the relocations might end up on odd addresses, which is not
representable by the soon to be added chained fixups.

This is arguably a bug in MC, but this behavior has been there since TLV
support was originally added.

This patch forces the `__thread_vars` sections to be aligned to the
target's pointer size. This is done by ld64 as well.

Differential Revision: https://reviews.llvm.org/D134594