xen/arm: livepatch: Prevent CPUs to fetch stale instructions after livepatching

xen/arm: livepatch: Prevent CPUs to fetch stale instructions after livepatching

During livepatch, a single CPU will take care of applying the patch and
all the others will wait for the action to complete. They will then once
execute arch_livepatch_post_action() to flush the pipeline.

Per B2.2.5 "Concurrent modification and execution of instructions" in
DDI 0487E.a, flushing the instruction cache is not enough to ensure new
instructions are seen. All the PEs should also do an isb() to
synchronize the fetched instruction stream.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Acked-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Reviewed-by: Volodymyr Babchuk <volodymyr_babchuk@epam.com>