SMP - fixed IPI livelock

- two CPUs can issue IPI to each other now without any hazzard

- we must be able to handle synchronous scheduling IPIs from
  other CPUs when we are waiting for attention from another one.
  Otherwise we might livelock.

- necessary barriers to prevent reordering

kernel/smp.h

@@ -73,6 +73,9 @@ _PROTOTYPE(void smp_schedule_migrate_proc,
 _PROTOTYPE(void arch_send_smp_schedule_ipi, (unsigned cpu));
 _PROTOTYPE(void arch_smp_halt_cpu, (void));
 
+/* deal with x-cpu scheduling event */
+_PROTOTYPE(void smp_sched_handler, (void));
+
 #endif /* __ASSEMBLY__ */
 
 #endif /* CONFIG_SMP */