x86: kill #ifdef for exit_idle()

Impact: cleanup

Introduce helper inline function in arch/x86/include/asm/idle.h
to remove #ifdefs around exit_idle().

Signed-off-by: Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>