scons: Use six.input instead of raw_input.

scons: Use six.input instead of raw_input.

raw_input is not defined in python 3.x and has been replaced by "input".
The "six" compatiblity module defines its own "input" method which
figures out which to use under the covers.

Change-Id: I13a885dd45ec0160c7b46e334b06aae239e3c836
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/27948
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>