Improve mc_arc() parameters

- Make the mc_arc() function declaration consistent with the definition
- isclockwise is supposed to be bool type, given how it is used.

Firmware/Marlin_main.cpp

@@ -9751,7 +9751,7 @@ void prepare_move()
   set_current_to_destination();
 }
 
-void prepare_arc_move(char isclockwise) {
+void prepare_arc_move(bool isclockwise) {
   float r = hypot(offset[X_AXIS], offset[Y_AXIS]); // Compute arc radius for mc_arc
 
   // Trace the arc