configure: arm: Fixes to build and run tests on Apple Silicon

On MacOS:

$> uname -m

returns:

arm64

To unify how we handle the achitecture detection across different
systems, sed it to aarch64 which is what's typically reported on
Linux.

In addition, when HVF is the acceleration method on aarch64, make sure
we select the right processor when invoking qemu.

Signed-off-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Signed-off-by: Andrew Jones <drjones@redhat.com>