scons: revert removal of -Werror for gem5 21.0
scons: revert removal of -Werror for gem5 21.0

This reverts:
https://gem5-review.googlesource.com/c/public/gem5/+/43425

Change-Id: Ic239150a7f2968744e40df40a6c03a942dc41ea6
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/43646
Reviewed-by: Bobby R. Bruce <b9dba908ffb6c6d8ffe449cc88b3709c13c485c2@ucdavis.edu>
Maintainer: Bobby R. Bruce <b9dba908ffb6c6d8ffe449cc88b3709c13c485c2@ucdavis.edu>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

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