x86: isolate the rtc code for sharing

The mach-default/mach_time.h code inline is moved to arch/x86/kernel/rtc.c
and the header files are adjusted.

Shrink the 3 dozen includes to the ones we really need.

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