[lld-macho] Ensure .tbss sections are also considered as ZeroFilled

This diff makes the behavior in {D80859} and {D81888} apply to
thread-local ZeroFill sections too. I realized this was necessary whie
trying to implement thread-local variables.

Reviewed By: #lld-macho, compnerd, MaskRay

Differential Revision: https://reviews.llvm.org/D85079