x86: Free per-cpu area for offline cpu via RCU.

x86: Free per-cpu area for offline cpu via RCU.

This allows other CPUs to reference per-cpu areas with less strict
locking. In particular, timer.c access a per-cpu lock with reference
to a per-timer cpu field which it accesses with no synchronisation.

One subtlety is that this prevents us bringing a cpu back online until
the RCU work is completed. In this case we return EBUSY and the
tool stack can report the (unlikely) error, or retry, as it sees fit.

Signed-off-by: Keir Fraser <keir@xen.org>