[lld-macho][nfc] Use %X in mapfile test

LLD (and ld64) emits uppercase hex addresses in the mapfile. The
map-file.s test passes right now because the addresses we emit happen
not to include any alphabets, but that can easily change.

I noticed this while dealing with
https://github.com/llvm/llvm-project/issues/54184.

Reviewed By: #lld-macho, thakis

Differential Revision: https://reviews.llvm.org/D120941