kernel/poll: Remove dummy waitq from stack

The poll code uses a dummy wait queue so the threads have something to
block on, but the previous coherence pass (which rearranged things to
put the _poller data elsewhere) missed that this was on the stack,
which is not allowed.  It actually has no use except as a list, so
make it a global static instead.

Signed-off-by: Andy Ross <[email protected]>

Author: Andy Ross

kernel/poll.c

@@ -284,7 +284,7 @@ int z_impl_k_poll(struct k_poll_event *events, int num_events,
 		return -EAGAIN;
 	}
 
-	_wait_q_t wait_q = Z_WAIT_Q_INIT(&wait_q);
+	static _wait_q_t wait_q = Z_WAIT_Q_INIT(&wait_q);
 
 	int swap_rc = z_pend_curr(&lock, key, &wait_q, timeout);