rt_mutex based ww_mutex implementation

[Sebastian Andrzej Siewior]

On -RT we use struct rt_mutex instead of struct mutex for a regular "mutex".
And since people use GPUs there, too we needed a port of ww_mutex based
rtmutex. I snipped the implementaton out of -RT tried to fit on mainline
and not breaking too much. It doesn't look that bad now :)
#1 is here to keep the changes in #2 a little smaller.

Not sure if #3 makes sense without #2 but it looks like this has been
forgotten.

Sebastian

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