aarch64: Add GCS support to longjmp

aarch64: Add GCS support to longjmp

This implementations ensures that longjmp across different stacks
works: it scans for GCS cap token and switches GCS if necessary
then the target GCSPR is restored with a GCSPOPM loop once the
current GCSPR is on the same GCS.

This makes longjmp linear time in the number of jumped over stack
frames when GCS is enabled.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>
Reviewed-by: Wilco Dijkstra <Wilco.Dijkstra@arm.com>