x86: use year 2000 offset for cmos clock

We know it is already after 2000. Use the year 2000 offset for both 32
and 64 bit, which removes ifdefs and the 1970 magic.

[ tglx@linutronix.de: remove 1970 magic, replace bogus commit message ]

Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>