single shot timer interrupts fix

single shot timer interrupts fix

- accidentaly this wasn't part of the SMP merge and the implementation
  remained uncomplete with the timer keeping ticking periodically

- APIC timer is set for a signel shot and restarted everytime it
  expires. This way we can keep the AP's trully idle

- the timer is restarted a little later before leaving to userspace

- LAPIC_TIMER_ICR is written before LAPIC_LVTTR so the newest value is
  used