util,scons: Generalize the aarch64 scons for the m5 util.

util,scons: Generalize the aarch64 scons for the m5 util.

Slightly parameterize it so it can be used with the other versions of
the utility.

All build products for a given variant will now go under
build/${VARIANT}. The primary build outputs will go under
build/${VARIANT}/out so that they're easy to distinguish.

Change-Id: Idd244cc2a6c08ec8e4d67de3d0bae604c0611220
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/27217
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: Gem5 Cloud Project GCB service account <345032938727@cloudbuild.gserviceaccount.com>
Tested-by: kokoro <noreply+kokoro@google.com>