[PATCH] x86-64: Avoid overflows during apic timer calibration

- Use 64bit TSC calculations to avoid handling overflow
- Use 32bit unsigned arithmetic for the APIC timer. This
way overflows are handled correctly.
- Fix exit check of loop to account for apic timer counting down

Signed-off-by:  <dpreed@reed.com>
Signed-off-by: Andi Kleen <ak@suse.de>