utils,tests: Enable passing of build args to compiler-tests.sh

utils,tests: Enable passing of build args to compiler-tests.sh

Previously we passed "-j `nproc`" to the scons. This a greedy approach
that should not be default. This change was introduced so the "-j" flag
may be passed via the "util/compiler-tests.sh" script.

Change-Id: I2e891ae3a9819770bd3ef15b95b81b7f5b71f7fa
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/31734
Reviewed-by: Hoa Nguyen <hoanguyen@ucdavis.edu>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>