kernel: expand fixed clock interrupt periods to 64-bit values

kernel: expand fixed clock interrupt periods to 64-bit values

Technically, all the current fixed clock interrupt periods fit within
an unsigned 32-bit value.  But 32-bit multiplication is an accident
waiting to happen.  So, expand the fixed periods for hardclock,
statclock, profclock, and roundrobin to 64-bit values.

One exception: statclock_mask remains 32-bit because random(9) yields
32-bit values.  Update the initclocks() comment to make it clear that
this is not an accident.