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 <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Gabe Black <5c8aa1266c03186393d9e7b3b4e3b14239af8201@google.com>
Maintainer: Gabe Black <5c8aa1266c03186393d9e7b3b4e3b14239af8201@google.com>

gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5