scons: Move some compiler flag setting code to the SConstruct.

scons: Move some compiler flag setting code to the SConstruct.

These settings are invariant, so there's no reason to apply them over
and over again for the child environments used for various build
products.

Change-Id: Icb4053105e4f1c43008f2422ba30c7206b7ff365
Reviewed-on: https://gem5-review.googlesource.com/5981
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>