powerpc64: trigger deferred DEC interrupts from splx(9)

powerpc64: trigger deferred DEC interrupts from splx(9)

In order to move to a machine-independent clock interrupt subsystem,
the powerpc64 clock interrupt code needs to work without knowing
anything about the clock interrupt schedule.

The easiest way to do this is, if the DEC fires while the CPU's IPL is
at or above IPL_CLOCK, to postpone clock interrupt work until the
clock interrupt is logically unmasked from splx(9).

Because we no longer defer work until the next tick, we don't need to
keep track of pending statclock ticks in the cpu_info struct.

With input from kettenis@.

Graciously compiled and tested by gkoehler@ and kettenis@.

Link: https://marc.info/?l=openbsd-tech&m=165862522102767&w=2
ok kettenis@ gkoehler@.