set the current CPU into idle state
MSOP_IDLE_SELF is called on a CPU that is otherwise
idle. It does not return until
is called from another CPU. The call to MSOP_IDLE_EXIT
may happen before, or at any point during the call to, MSOP_IDLE_SELF.
In this case, MSOP_IDLE_SELF returns immediately, and does not
wait for a further call to MSOP_IDLE_EXIT.