[lld/mac] Test zerofill sections after __thread_bss

Real zerofill sections go after __thread_bss, since zerofill sections
must all be at the end of their segment and __thread_bss must be right
after __thread_data.

Works fine already, but wasn't tested as far as I can tell.

Also tweak comment about zerofill sections a bit.

No behavior change.

Differential Revision: https://reviews.llvm.org/D104609