sched: Create sched_select_non_idle_cpu() to give preferred CPU for power saving

In order to save power, it would be useful to schedule light weight work on cpus
that aren't IDLE instead of waking up an IDLE one.

By idle cpu (from scheduler's perspective) we mean:
- Current task is idle task
- nr_running == 0
- wake_list is empty

This is already implemented for timers as get_nohz_timer_target(). We can figure
out few more users of this feature, like workqueues.

This patch converts get_nohz_timer_target() into a generic API
sched_select_non_idle_cpu() so that other frameworks (like workqueue) can also
use it.

This routine returns the cpu which is non-idle. If the local CPU isn't idle OR
all cpus are idle, local cpu is returned back. If local cpu is idle, then we
must look for another CPU which isn't idle.

This patch reuses the code from get_nohz_timer_target() routine, which had
similar implementation.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Seongmin Park <r_data@naver.com>