rt_thread_handle_sig function will clean signal bit in state of thread
always.

Disable interrupt to check thread->stat in _signal_entry to ensure the
signal is not lost.

At the same time, remove the disable interrupt return code from
rt_thread_handle_sig function.

[kernel]Change 'rt_timer_timeout_hook' function to 'rt_timer_enter_hook' and add 'rt_timer_exit_hook' hook function.

Update license for RT-Thread kernel.