Rendezvous selected cpus in softirq (stop_machine).

Rendezvous selected cpus in softirq (stop_machine).

This is similar to stop_machine_run stub from Linux, to pull
selected cpus in rendezvous point and the do some batch work
under a safe environment. Current one usage is from S3 path,
where individual cpu is pulled down with related online
footprints being cleared. It's dangerous to have other cpus
checking clobbered data structure in the middle, such as
cpu_online_map, cpu_sibling_map, etc.

Signed-off-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Keir Fraser <keir.fraser@citrix.com>