improve futex on -RT by avoiding the double wake-up

[Sebastian Andrzej Siewior]

On -RT we see a double wake up if the waiter has a higher priority than
the process doing the wake up. Patch #1 avoids in the PI wake code, #2 in
the non-PI variant (and only for one waiter).

#3 is a small optimisation for message queues avoid a "cpu_relax()" loop.

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