kernel/poll: Remove POLLING thread state bit

The _THREAD_POLLING bit in thread_state was never actually a
legitimate thread "state".  It is a clever synchronization trick
introduced to allow the thread to release the irq_lock while looping
over the input event array without dropping events.

Instead, make that flag a word in the "poller" struct that lives on
the stack of the thread calling k_poll.  The disadvantage is the 4
bytes of thread space needed.  Advantages:

+ Cleaner API, it's now internal to poll instead of being globally
  visible.

+ The thread_state bit space is just one byte, and was almost full
  already.

+ Smaller code to write/test a full word and not a bitfield

+ Words are atomic, so no need for one of irq lock/unlock pairs.

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

include/kernel.h

@@ -4191,6 +4191,7 @@ extern void *k_calloc(size_t nmemb, size_t size);
 /* private - implementation data created as needed, per-type */
 struct _poller {
 	struct k_thread *thread;
+	volatile int is_polling;
 };
 
 /* private - types bit positions */