scons: Update the Check(Cxx|Link)Flag checks to set their vars.

scons: Update the Check(Cxx|Link)Flag checks to set their vars.

This avoids boilerplate where we check to see if flag X is supported,
and if so then set flag X. Since there are shared and static versions of
the linker flags but we only explicitly check the static ones, this
change also adds a parameter to CheckLinkFlag to set both flavors. This
defaults to true since I assume most of the time linking flags will
apply to both.

Change-Id: I983222169e9835aeb98570362f7004e2ef0240d0
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/40855
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>