kernel: poll: Lock all calls to signal_poll_event

The signal_poll_event function was previously called without the poll
lock held. This created a race condition between a thread calling k_poll
to wait for an event and another thread signalling for this same event.
This resulted in the waiting thread to stay pending and the handle to it
getting removed from the notifyq, meaning it couldn't get woken up
again.

Signed-off-by: Ambroise Vincent <ambroise.vincent@arm.com>