scons,python: Update makeTheGPUISA to Python3

scons,python: Update makeTheGPUISA to Python3

This function was causing an error to occur when trying to compile HSAIL
and GCN in a Python3 environment. It has now been upgraded to work in
both Python2 and Python3.

Change-Id: If8d6ee1e08c47d5a36182afc10cf86a8e905bda0
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/29410
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>