arm64: initial drop

This is the initial drop of the arm64 test framework and a first test
that just checks that setup completed (a selftest). kvm isn't needed
to run this test unless testing with smp > 1.

Try it out with
  yum install gcc-aarch64-linux-gnu
  ./configure --cross-prefix=aarch64-linux-gnu- --arch=arm64
  make
  QEMU=[qemu with aarch64, mach-virt, and chr-testdev] ./run_tests.sh

Signed-off-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>