hppa: switch to clockintr

hppa: switch to clockintr

- Remove hppa-specific clock interrupt scheduling bits from cpu_info.
- Rename cpu_hardclock() to itmr_intr(); it doesn't exclusively run
  hardclock(9) anymore.
- Wire up itmr_intrclock.

hppa now has a randomized statclock(), stathz = hz.

Patch help, testing, and review from kettenis@ (B2000) and miod@ (C3650).
MP testing from guenther@ (dual-core J6700).

ok mlarkin@ kettenis@, "Ship it?" guenther@