scons: Simplify check for have_posix_clock.

scons: Simplify check for have_posix_clock.

The "library" argument can actually be a list, in which case each
element of the list is checked, one after the other. Use that rather
than calling the CheckLibWithHeader method multiple times and manually
oring the results.

Change-Id: I5c774be15eaa9a7e52ec3ee5a1bbcc5ef1fa0f41
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/40863
Tested-by: kokoro <noreply+kokoro@google.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>