x86_64: prepare idle loop for dynamic ticks

Add tick_nohz_{stop,restart}_sched_tick to idle loop in prepartion for turning
on dynticks.  These are just noops until NO_HZ is enabled.

Signed-off-by: Chris Wright <chrisw@sous-sol.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>