Fix a hole in the scheduler where collisions between waking blocked threads in mutexe...

Fix a hole in the scheduler where collisions between waking blocked threads in mutexes with interrupts waking blocked threads in message queues can occur. Queue posts will put the threads on a separate list that is then added to the running list with IRQs disabled on the next task switch or CPU wakeup. Basically no overhead for other operations. Seems a likely cause of my occasional observation of the backlight fade causing playback threads to stop running and a recently reported blocking violation upon USB plugging. Time will tell but banging the backlight on and off frequently hasn't hiccuped again for me on H120.

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@12915 a1c6a512-1295-4272-9138-f99709370657