Add a simpler version of spinlock support for ARM64 for
the guests to use.

The implementation is loosely based on the spinlock
implementation in kvm-unit-tests.
Signed-off-by: NRaghavendra Rao Ananta <rananta@google.com>
Reviewed-by: NOliver Upton <oupton@google.com>
Reviewed-by: NAndrew Jones <drjones@redhat.com>
Signed-off-by: NMarc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20211007233439.1826892-12-rananta@google.com