[lld/mac] Make sure __thread_ptrs is in front of __thread_bss

The exact location doesn't matter, but it should be in front
of __thread_bss. We put it right in front of __thread_data
which is where ld64 seems to put it as well.

Fixes PR50769.

(As mentioned on the bug, there is probably a more structural
fix too, see comment 5. If we don't address this, it's likely
we'll run into this again with other synthetic sections. But
for now, let's fix the immediate breakage.)

Differential Revision: https://reviews.llvm.org/D104596