[lld-macho][test][nfc] Update stabs.s to use GMT time zone instead of UTC

This is to work around a singularity container issue we ran into recently. The
container fails to honor the time zone setting (TZ=UTC) when executing the touch
command. Replacing UTC with GMT worked correctly in the container. This change
does not change (at least not intentionally) the function of the test in any way
since GMT and UTC should be equivalent AFAIK. Please refer to the discussions in
D139980 for more background information.

Reviewed By: int3, #lld-macho

Differential Revision: https://reviews.llvm.org/D140233