clockintr: prepare to hoist clockqueue_init() out of clockintr_cpu_init()

clockintr: prepare to hoist clockqueue_init() out of clockintr_cpu_init()

Reorganize the initialization block in clockintr_cpu_init() so that it
doesn't break when clockqueue_init() is called separately:

- If CQ_INTRCLOCK is not set, this is the first clockintr_cpu_init()
  call and we can install the intrclock given as argument.

- If any of the internal clock interrupt handles are NULL, this is the
  first clockintr_cpu_init() call and we need to establish them.