kernel/sched: Optimize deadline comparison

Needing to check the current cycle time (which involves a spinlock and
register read on most architectures) is wasteful in the scheduler
priority predicate, which is a hot path.  If we "burn" one bit of
precision (and document the rule), we can do the comparison without
knowing the current time.

2^31 cycles is still far longer than a live deadline thread in any
legitimate realtime app should ever live before being scheduled.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>

include/kernel.h

@@ -718,11 +718,12 @@ __syscall void k_thread_priority_set(k_tid_t thread, int prio);
  * static priority.  Threads at different priorities will be scheduled
  * according to their static priority.
  *
- * @note Deadlines that are negative (i.e. in the past) are still seen
- * as higher priority than others, even if the thread has "finished"
- * its work.  If you don't want it scheduled anymore, you have to
- * reset the deadline into the future, block/pend the thread, or
- * modify its priority with k_thread_priority_set().
+ * @note Deadlines are stored internally using 32 bit unsigned
+ * integers.  The number of cycles between the "first" deadline in the
+ * scheduler queue and the "last" deadline must be less than 2^31 (i.e
+ * a signed non-negative quantity).  Failure to adhere to this rule
+ * may result in scheduled threads running in an incorrect dealine
+ * order.
  *
  * @note Despite the API naming, the scheduler makes no guarantees the
  * the thread WILL be scheduled within that deadline, nor does it take