score: Remove idle field of Per_CPU_Control

This field is unused except for special case simulator clock drivers.
In these places use an alternative.  Add and use
_Thread_Set_global_exit_status() and _Thread_Get_global_exit_status().

c/src/lib/libcpu/powerpc/mpc6xx/clock/c_clock.c

@@ -98,8 +98,11 @@ static void clockHandler(void)
   #if (CLOCK_DRIVER_USE_FAST_IDLE == 1)
     do {
       rtems_clock_tick();
-    } while ( _Thread_Executing == _Thread_Idle &&
-              _Thread_Heir == _Thread_Executing);
+    } while (
+      _Thread_Heir == _Thread_Executing
+        && _Thread_Executing->Start.entry_point
+          == rtems_configuration_get_idle_task()
+    );
 
   #else
     rtems_clock_tick();