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/utility.c

@@ -33,9 +33,12 @@ PUBLIC void panic(const char *fmt, ...)
 	printf("\n");
   }
 
-  printf("kernel: ");
+  printf("kernel on CPU %d: ", cpuid);
   util_stacktrace();
 
+  printf("current process : ");
+  proc_stacktrace(get_cpulocal_var(proc_ptr));
+
   /* Abort MINIX. */
   minix_shutdown(NULL);
 }