SMP - Only a single APIC timer handler

- bsp_timer_int_handler() and ap_timer_int_handler() unified into
  timer_int_handler()

- global realtime updated only on BSP

kernel/arch/i386/include/arch_clock.h

@@ -1,6 +1,8 @@
 #ifndef __CLOCK_X86_H__
 #define __CLOCK_X86_H__
 
+#include "../apic_asm.h"
+
 _PROTOTYPE(int init_8253A_timer, (unsigned freq));
 _PROTOTYPE(void stop_8253A_timer, (void));