arm64: switch to clockintr(9)

arm64: switch to clockintr(9)

Switch arm64 to the clockintr(9) subsystem.

- Remove the custom per-CPU clock interrupt schedule from agtimer(4).
- Remove the custom randomized statclock() pieces from agtimer(4).
- Add agtimer_rearm(), agtimer_trigger(), and wire up agtimer_intrclock.

There is one wart:

- The AArch64 spec says that a value written to CNTV_TVAL_EL0 is
  "treated as a signed 32-bit integer" [1].  kettenis@ doesn't know
  what to make of this.  I'm capping the value at INT32_MAX for
  now.  It's possible I am misreading this, though.

Tested by kettenis@ on his Apple M1 mini.  Tested by me on my
Raspberry Pi 4B.

Link: https://marc.info/?l=openbsd-tech&m=166776342503304&w=2
[1] "Arm Architecture Reference Manual for A-profile architecture"
    issue I.a, section D17.11.27 ("CNTV_TVAL_EL0").

ok kettenis@