fix the bug when look up for new process Index will overflow

Signed-off-by: Le Min <lemin9538@gmail.com>
enable os_tick_handler and task can be switch via timer irq

need save lr and spsr register of each mode, if not this will
cause system run without controller. next step, try to figure
out a good algo of schedule and optimal the code style.

Signed-off-by: Le Min <lemin9538@gmail.com>