sched/fair: Update signals of nohz cpus if we are going idle

sched/fair: Update signals of nohz cpus if we are going idle

Stale cpu utilization signals can cause havoc for energy-aware systems,
and they are caused by no updates being performed for cpus which have
no tick running. There is open debate about when is the correct time to
update these cpus, and general recognition that something needs to be
done.

This is an attempt to do something useful.

When we are looking for a task to pull for a newly-idle cpu, we have
an opportunity to update the stats for any cpu which has no tick running
without causing too much disturbance to the system or waking it up.

Change-Id: I0280104ea9c53e56c26f1c56a62bacab5d3e951b
Signed-off-by: Chris Redpath <chris.redpath@arm.com>
Signed-off-by: Brendan Jackman <brendan.jackman@arm.com>