gptimer(4): switch to clockintr

gptimer(4): switch to clockintr

- Remove custom clock interrupt scheduling code.
- Remove local evcount structs.
- Wire up gptimer_intrclock.
- Switch stathz from 128 to hz
- Switch profhz from 1024 to (stathz * 10).

This change is untested.  Nobody seems to have hardware that actually uses
the gptimer(4) as an interrupt clock.  If this patch doesn't work, the driver
is probably not too distant from a working state.

Compile-tested by jca@.  Discussed with kettenis@, jca@, drahn@, patrick@,
jsg@, and uwe@.

Link: https://marc.info/?l=openbsd-tech&m=167451333419815&w=2
ok patrick@ kettenis@