Allow one shot timer to be switched on a running system between i8254 and

Allow one shot timer to be switched on a running system between i8254 and
lapic timer:

- Always register "clk" interrupt.
- Add cputimer_intr_switch(), which could switch one shot timer between
  i8254 and lapic timer on a running system.  It could be used to select
  a proper one shot timer duing ACPI C3 transition:
  e.g. ->C3 use i8254, C3-> use lapic timer
- Add sysctl node hw.cputimer_intr_type to test cputimer_intr_switch().