clockintr_cpu_init: mask CQ_INTRCLOCK while advancing schedule

clockintr_cpu_init: mask CQ_INTRCLOCK while advancing schedule

Allowing the intrclock to fire in the midst of clockintr_cpu_init()
would complicate the function a lot.  However, in a future patch we
will need to enable intrclock operations in clockintr_advance(),
clockintr_cancel(), and clockintr_schedule().

We can avoid this conflict by masking CQ_INTRCLOCK while we're
updating the internal clockintrs in clockintr_cpu_init().  When we no
longer need clockintr_cpu_init(), this workaround will disappear.