sched: find the latest idle cpu

Currently we just try to find least load cpu. If some cpus idled,
we just pick the first cpu in cpu mask.

In fact we can get the interrupted idle cpu or the latest idled cpu,
then we may get the benefit from both latency and power.
The selected cpu maybe not the best, since other cpu may be interrupted
during our selecting. But be captious costs too much.

Signed-off-by: Alex Shi <alex.shi@linaro.org>