unified: Add _is_next_thread_current()

The routine _is_next_thread_current() checks if the current thread
is still the highest priority thread that is ready to run.

It is useful for determining if a thread must be swapped out when
timeslicing is in effect.

Change-Id: Ide7b89742a64f6082ca4c679a4b2fbd60792e30f
Signed-off-by: Peter Mitsis <peter.mitsis@windriver.com>