configs: Remove Python 2 compatibility code in Arm configs

configs: Remove Python 2 compatibility code in Arm configs

Remove uses of six and imports from __future__ and use native Python 3
functionality instead.

Change-Id: If37718ba99def2d6f176604e20d4ebeda75474ad
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/39581
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Richard Cooper <richard.cooper@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>