Do a bit of rude hackery to get clock interrupts on all CPUs. This
is partly based on the Alpha system which duplicates the clock to
each cpu, instead of doing a clock roundrobin like on i386. This means
we get hz * ncpu clocks per second and so we have to seperate clock
sampling from actual 'do the work' clock processing. The BSP runs the
complete processing, the rest just sample state etc.

Using the on-cpu interval timer is not ideal as it will drift. There
is more to be done here, we should use an external clock source.