[lld-macho][test] Rework map-file.s

Merge the checks done on the `c-string-literals` binary with those on
the main `test` binary. Also switch some checks to `-DAG` instead of
`-NEXT`. I'm about to extend this test, and this is easier to work with.

Reviewed By: #lld-macho, oontvoo

Differential Revision: https://reviews.llvm.org/D135999