scons: Add a mechanism to accumulate warnings to reprint at the end.

scons: Add a mechanism to accumulate warnings to reprint at the end.

When building gem5, it's possible for warnings printed early in the
build to be quickly wisked away in a see of compile lines, never to be
seen again (or driven off the end of the scrollback buffer).

To avoid those messages getting lost or ignored, this change adds a
mechanism to aggregate them into a list so that they can be summarized
at the end of the build, successful or not.

Change-Id: Ie13320717698fcbcd3a8f8d1c062467e8d6d2914
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/27129
Tested-by: Gem5 Cloud Project GCB service account <345032938727@cloudbuild.gserviceaccount.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Gabe Black <gabeblack@google.com>