scons: Call summarize_warnings() when scons finishes building.

scons: Call summarize_warnings() when scons finishes building.

This will ensure that warnings are not all shoved off the end of the
scrollback buffer or lost in a sea of compiler lines, and that the user
will actually have a chance to see and read them.

Change-Id: I7129560482ebca903ec597f8b1cf8a9a84d98c9c
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/27130
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>