[lld-macho][test][nfc] Update stabs.s to use touch -d instead of -t

The test currently uses `touch -t` (e.g., `env TZ=UTC touch -t
"197001010000.16"`) to set file timestamps. However, this does not seem to set
the time zone correctly in a singularity container. While this is probably a
bug/limitation of the singularity container, but we can instead use `touch
-d` (e.g., `touch -d "1970-01-01 00:00:16 UTC"`) to achieve the same result
without relying on a fix from singularity. Thoughts?

Reviewed By: int3, #lld-macho

Differential Revision: https://reviews.llvm.org/D139980