kernel: Update k_wakeup()

This commit does two things to k_wakeup():

1. It locks the scheduler before marking the thread as not suspended.
As the the clearing of the _THREAD_SUSPENDED bit is not atomic, this
helps ensure that neither another thread nor ISR interrupts this
action (resulting in a corrupted thread_state).

2. The call to flag_ipi() has been removed as it is already being
made within ready_thread().

Signed-off-by: Peter Mitsis <peter.mitsis@intel.com>