util: Add a --verbose flag to the m5 util's scons.

util: Add a --verbose flag to the m5 util's scons.

Like gem5's own verbose scons flag, when this isn't provided, the output
is very brief and just shows what is being built and by what type of
process. When it is provided, the full command lines are printed.

This is less fancy than the version gem5 has, but I didn't want to
duplicate all that code. We should find a way to share that and other
functionality between different sets of scons scripts.

Change-Id: Id9973b57a1270ec8b364efd2aa67d49b0fb82a9d
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/27756
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>