scons: Simplify kvm architecture compatibility check.

scons: Simplify kvm architecture compatibility check.

The original implementation had a function we'd call back into later
which checked if a given ISA has KVM support on the current host.
Instead, this change reverse that and statically figures out which
single target ISA could possibly run under KVM on this host. Then later,
we can just check if the ISA we're trying to build is that one.

Change-Id: I3e7e06180983dfcc4611181718eb5d6210844d36
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/40870
Tested-by: kokoro <noreply+kokoro@google.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>