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

[pzheng]

Summary

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.

[int3]

Let's try it. Please watch the buildbots when it lands & fast revert if necessary

[pzheng]

Sure, I'll monitor the buildbot in case this causes any failures.

[thakis]

As requested on the other review, please also report this problem upstream to your container vendor, so that eventually workarounds like this are hopefully no longer needed.

[pzheng]

Sure, @thakis. We are working with our IT department to get to the bottom of this and will report the issue to the container vendor if IT concludes it's a container bug.

lld/test/MachO/stabs.s

 # REQUIRES: x86, shell
 # UNSUPPORTED: system-windows
 # RUN: rm -rf %t; split-file %s %t
 # RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %t/test.s -o %t/test.o
 # RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %t/foo.s -o %t/foo.o
 # RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %t/no-debug.s -o %t/no-debug.o
 ## Set modtimes of the files for deterministic test output.
-# RUN: env TZ=UTC touch -t "197001010000.16" %t/test.o
-# RUN: env TZ=UTC touch -t "197001010000.32" %t/foo.o
+# RUN: env TZ=GMT touch -t "197001010000.16" %t/test.o
+# RUN: env TZ=GMT touch -t "197001010000.32" %t/foo.o
 # RUN: llvm-ar rcsU %t/foo.a %t/foo.o
 
 # RUN: ZERO_AR_DATE=0 %lld -lSystem %t/test.o %t/foo.o %t/no-debug.o -o %t/test
 # RUN: (llvm-objdump --section-headers %t/test; dsymutil -s %t/test) | \
 # RUN:   FileCheck %s -DDIR=%t -DFOO_PATH=%t/foo.o \
 # RUN:       -D#TEST_TIME=0x10 -D#FOO_TIME=0x20
 
 ## Check that we emit the right modtime even when the object file is in an