[lld/mac] Add support for segment$start$ and segment$end$ symbols

These symbols are somewhat interesting in that they create non-existing
segments, which as far as I know is the only way to create segments
that don't contain any sections.

Final part of part of PR50760. Like D106629, but for segments instead
of sections. I'm not aware of anything that needs this in practice.

Differential Revision: https://reviews.llvm.org/D106767