Add external closed loop controller support

Marlin/src/gcode/gcode.cpp

@@ -292,6 +292,10 @@ void GcodeSuite::process_parsed_command(
         case 5: M5(); break;                                      // M5 - turn spindle/laser off
       #endif
 
+      #if ENABLED(EXTERNAL_CLOSED_LOOP_CONTROLLER)
+        case 12: M12(); break;                                    // M12: Synchronize and optionally force a CLC set
+      #endif
+
       case 17: M17(); break;                                      // M17: Enable all stepper motors
 
       #if ENABLED(SDSUPPORT)