powerpc, macppc: switch to clockintr

powerpc, macppc: switch to clockintr

- Remove powerpc-specific clock interrupt scheduling bits from cpu_info.
- Remove macppc-specific randomized statclock() bits from macppc/clock.c.
- Remove the 'stat_count' evcount.  All clock interrupts are now counted
  via the 'clock_count' evcount.
- Wire up dec_intrclock.

Bringup help from gkoehler@.  The patch has survived five or six
kernel-release-upgrade cycles on my dual-core PowerMac3,6.

Link: https://marc.info/?l=openbsd-tech&m=166776385003520&w=2
ok gkoehler@ mlarkin@