This brings the x86-64 port uptodate.  Only architecture specific
changes.

The biggest change is the forward port of the 2.4 timing code with full
HPET support.  This should improve timing stability on some Opteron
boxes considerably.

Also add the optimized low level functions from 2.4 (clear_page,
copy_page, memcpy, csum_copy etc.) They were supposed to be merged
earlier, but got dropped due to some SNAFU.  Especially the clear_page
changes should improve performance considerably, because the old version
used write combining writes which put all the new process data out of
cache.  New version serves cache hot.

Also some other bugfixes.

Full changelog:
- Re-add some lost patches: improved copy_page, clear_page, memset, memcpy,
  csum_copy from 2.4.
- New timing code from 2.4 (Bryan O'Sullivan, John Stultz, Vojtech Pavlik)
- Use correct MSR to write northbridge MCE configuration
- Fix and reenable simics check in APIC timer calibration
- Check if BIOS enabled APIC and don't use APIC mode if not.
- Remove some obsolete code in APIC handling.
- Fix potential races in the IOMMU code.
- Don't print backtrace twice on oops.
- Fix compilation of swsuspend (Pavel Machek)
- Add oops locking to kernel page faults.
- Use prefetcht0 for C level kernel prefetches.