kernel: sched: Optimize sleeping function

Accessing system timer registers can be costly and it shall be avoided
if possible. When thread is waken up in z_tick_sleep it may be because
timeout expired or because thread was waken up before sleeping period
passed.

Add function to detect if timeout is aborted (before it was expired).
Use it in the sleep function and avoid reading system ticks if timeout
was not aborted.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>