kernel: SMP-aware scheduler

The scheduler needs a few tweaks to work in SMP mode:

1. The "cache" field just doesn't work.  With more than one CPU,
   caching the highest priority thread isn't useful as you may need N
   of them at any given time before another thread is returned to the
   scheduler.  You could recalculate it at every change, but that
   provides no performance benefit.  Remove.

2. The "bitmask" designed to prevent the need to individually check
   priorities is likewise dropped.  This could work, but in fact on
   our only current SMP system and with current K_NUM_PRIOPRITIES
   values it provides no real benefit.

3. The individual threads now have a "current cpu" and "active" flag
   so that the choice of the next thread to run can correctly skip
   threads that are active on other CPUs.

The upshot is that a decent amount of code gets #if'd out, and the new
SMP implementations for _get_highest_ready_prio() and
_get_next_ready_thread() are simpler and smaller, at the expense of
having to drop older optimizations.

Note that scheduler synchronization is unchanged: all scheduler APIs
used to require that an irq_lock() be held, which means that they now
require the global spinlock via the same API.  This should be a very
early candidate for lock granularity attention!

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

include/kernel.h

@@ -392,6 +392,17 @@ struct _thread_base {
 		u16_t preempt;
 	};
 
+#ifdef CONFIG_SMP
+	/* True for the per-CPU idle threads */
+	u8_t is_idle;
+
+	/* Non-zero when actively running on a CPU */
+	u8_t active;
+
+	/* CPU index on which thread was last run */
+	u8_t cpu;
+#endif
+
 	/* data returned by APIs */
 	void *swap_data;