[lld-macho] Put GOT into `__DATA` segment where appropriate

We were previously always emitting the GOT into `__DATA_CONST`, even for
target platforms where it should end up in `__DATA`.

I stumbled onto this while trying to use the `class-dump` tool -- with
the wrong segment names, it fails to locate the ObjC runtime info and
therefore fails to dump any classes.

Reviewed By: #lld-macho, oontvoo

Differential Revision: https://reviews.llvm.org/D112500