x86_64: Generic timekeeping for x86_64

Hey Andi,
	Just wanted to send this second pass on the x86_64 generic 
timekeeping conversion. It includes a number of changes you suggested, 
however its possible I missed a few things. I've made sure the patchset 
compiles at each stage, and atleast w/ the box I was using it booted 
each step as well.

Still on the TODO:
o See about merging i386/x86-64 hpet.c (maybe driver/char/hpet.c as 
well?)
o 64bit hpet (should be trivial)
o Further cleanups

If anyone else is interested here, my full timekeeping tree can be 
found here: http://sr71.net/~jstultz/tod/

New in the current C6 release:
o x86-64 cleanups
o arch-trivial patch that converts arches w/o inter-tick resolution
o ia64 fixups by Peter Keilty

Let me know if you have any thoughts or comments!

thanks again!
-john