x86_64: enable clockevents and dynticks

This series converts x86_64 timers to clockevents drivers
and then enables dynticks.  There's some minor cleanups along
the way.  The lapic broadcast mechanism is untested, I'm sure it
still needs work, there's still some cruft in lapic_setup_timer.

This is just for comments at this point, now that it's working
on my test box in both NO_HZ=n and NO_HZ=n configurations (typically
using hpet).

thanks,
-chris
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