sched: wake_lock()

This adds a facility to wake a thread, but with the intention that it will
acquire a certain lock after waking, and while the waker holds the lock.
This is implemented using the regular wait morphing code (send_lock() and
receive_lock()), but with additional mutual exclusion to allow regular
wake()s in parallel.

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>