scons: Move a displaced have_posix_clock check back where it goes.

scons: Move a displaced have_posix_clock check back where it goes.

This check had been separated from the code handling have_posix_clock by
some intervening code doing something else. Bring the check back
alongside the code setting that variable.

Change-Id: I6acb54fddbb5c41d6c38d4b93e649835a4775fa0
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/40862
Tested-by: kokoro <noreply+kokoro@google.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>