scons: Pull Configure() to earlier in SConstruct so we can use it more.

scons: Pull Configure() to earlier in SConstruct so we can use it more.

This mechanism lets us check if headers are available, flags are
supported, etc. We should use that more often, rather than checking for
specific versions of tools where problematic new warnings were added, etc.

Change-Id: I5b1a6499147f27cc8944fcb8c61eb69e9fa8fb7a
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/40860
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>