util: Add SCons build files for the aarch64 verson of the m5 utility.

util: Add SCons build files for the aarch64 verson of the m5 utility.

These are currently specific to aarch64, but will be expanded to cover
all other versions of the utility as well.

The intention of these new files is to centralize the build mechanism
for the different versions of the utility so that they have consistent
features, mechanisms, and targets, and so that new features will
automatically be shared by all versions without having to be implemented
in each.

This also sets up a separate build directory which will keep the source
tree clean, and will (with some more development) make it possible to
build multiple versions of the m5 utility at the same time without them
running into each other.

Change-Id: I10018eef6beb4af30a8d3bbab8b82cabd2b3f22c
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/27213
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>