Fix an extremely subtle deadlock bug on x86_64
The recent_activity flag was an unsigned int, but we sometimes do a 64-bit xchg() on it, which overwrites the next word in memory. This happened to contain the sched_state flag, which is used to control the orderly shutdown of the system. If the xchg() happened during shutdown, the scheduler would get confused and deadlock. Don't you just love C?
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