sched: x86, track TSC-unstable events

track TSC-unstable events and propagate it to the scheduler code.
Also allow sched_clock() to be used when the TSC is unstable,
the rq_clock() wrapper creates a reliable clock out of it.

Signed-off-by: Ingo Molnar <mingo@elte.hu>

include/linux/sched.h

@@ -1321,6 +1321,8 @@ extern void sched_exec(void);
 #define sched_exec()   {}
 #endif
 
+extern void sched_clock_unstable_event(void);
+
 #ifdef CONFIG_HOTPLUG_CPU
 extern void idle_task_exit(void);
 #else