scons: Use conf to determine if some flags are supported.

scons: Use conf to determine if some flags are supported.

Use conf rather than checking for particular combinations of OS and/or
compiler versions. This *should* be just as correct, and not require
keeping track of what versions of the tools particular flags were added.

Change-Id: Icaa4c273fe89801e2808e6a814ce476700fd6956
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/40861
Tested-by: kokoro <noreply+kokoro@google.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>