Switch to the event timers infrastructure. This includes:

o   Setting td_intr_frame to the XIVs trap frame because it's referenced
    by the ET event handler.
o   Signal EOI to the CPU before calling the registered XIV handlers.
    This prevents lost ITC interrupts, which cause starvation in one-shot
    mode.
o   Adding support for IPI_HARDCLOCK with corresponding per-CPU counters.
o   Have the APs call cpu_initclocks() so as to limited the scattering of
    clock related initialization. cpu_initclocks() calls the <self>_bsp()
    or <self>_ap() version accordingly.
o   Uncomment the ET clock handling in cpu_idle().
o   Update the DDB 'show pcpu' output for the new MD fields.
o   Entirely rewritten ia64_ih_clock(). Note that we don't create as many
    clock XIVs as we have CPUs, as is done on PowerPC. It doesn't scale.
    We can only have 240 XIVs and we can have more CPUs than that. There's
    a single intrcnt index for the cumulative clock ticks and we keep per
    CPU counts in the PCPU stats structure.
o   Register the ITC by hooking SI_SUB_CONFIGURE (2nd order).

Open issues:
o   Clock interrupts can still be lost. Some tweaking is still necessary.

Thanks to: mav@ for his support, feedback and explanations.

ET stats while committing:
eris% sysctl machdep.cpu | grep nclks

machdep.cpu.0.nclks: 24007
machdep.cpu.1.nclks: 22895
machdep.cpu.2.nclks: 13523
machdep.cpu.3.nclks: 9342
machdep.cpu.4.nclks: 9103
machdep.cpu.5.nclks: 9298
machdep.cpu.6.nclks: 10039
machdep.cpu.7.nclks: 9479
eris% vmstat -i | grep clock
clock                      108599         50

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