scons: Removed -Werror for the gem5 20.1 release

scons: Removed -Werror for the gem5 20.1 release

While gem5 compiles on all our supported compilers, removing the -Werror
flag on the stable branch ensures that, as new compilers are released
with stricter warnings, gem5 remains compilable.

Change-Id: I9a356472dc4d729a3fef9f1455814c900103bd66
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/35455
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>