score: Simplify _Watchdog_Tick()

Move thread dispatch disable check to legacy rtems_clock_tick().  Assert
that thread dispatching is disabled in _Watchdog_Tick().  This is
usually the case, since this function is called in interrupt context by
the clock tick service routine.

cpukit/rtems/src/clocktick.c

@@ -20,6 +20,7 @@
 
 #include <rtems/rtems/clock.h>
 #include <rtems/score/timecounter.h>
+#include <rtems/score/threadimpl.h>
 
 rtems_status_code rtems_clock_tick( void )
 {
@@ -32,5 +33,9 @@ rtems_status_code rtems_clock_tick( void )
     &lock_context
   );
 
+  if ( _Thread_Dispatch_is_enabled() ) {
+    _Thread_Dispatch();
+  }
+
   return RTEMS_SUCCESSFUL;
 }

cpukit/score/src/watchdogtick.c

@@ -12,8 +12,9 @@
  * http://www.rtems.org/license/LICENSE.
  */
 
+#include <rtems/score/assert.h>
 #include <rtems/score/schedulerimpl.h>
-#include <rtems/score/threadimpl.h>
+#include <rtems/score/threaddispatch.h>
 #include <rtems/score/todimpl.h>
 #include <rtems/score/watchdogimpl.h>
 
@@ -23,12 +24,11 @@
 
 void _Watchdog_Tick( void )
 {
+  _Assert( !_Thread_Dispatch_is_enabled() );
+
   _TOD_Tickle_ticks();
 
   _Watchdog_Tickle_ticks();
 
   _Scheduler_Tick();
-
-  if ( _Thread_Dispatch_is_enabled() )
-    _Thread_Dispatch();
 }