Mark unchanging arrays as const (PR#2494)

The compiler may be able to optimize if it knows an array won’t be
changing.

Marlin/Marlin.h

   #define CRITICAL_SECTION_END    SREG = _sreg;
 #endif
 
-extern float homing_feedrate[];
 extern bool axis_relative_modes[];
 extern int feedrate_multiplier;
 extern bool volumetric_enabled;

Marlin/Marlin_main.cpp

 static int commands_in_queue = 0;
 static char command_queue[BUFSIZE][MAX_CMD_SIZE];
 
-float homing_feedrate[] = HOMING_FEEDRATE;
+const float homing_feedrate[] = HOMING_FEEDRATE;
 bool axis_relative_modes[] = AXIS_RELATIVE_MODES;
 int feedrate_multiplier = 100; //100->1 200->2
 int saved_feedrate_multiplier;
 #endif
 
 #ifdef SERVO_ENDSTOPS
-  int servo_endstops[] = SERVO_ENDSTOPS;
-  int servo_endstop_angles[] = SERVO_ENDSTOP_ANGLES;
+  const int servo_endstops[] = SERVO_ENDSTOPS;
+  const int servo_endstop_angles[] = SERVO_ENDSTOP_ANGLES;
 #endif
 
 #ifdef BARICUDA

Marlin/ultralcd.cpp

   #if HAS_POWER_SWITCH
     extern bool powersupply;
   #endif
-  static float manual_feedrate[] = MANUAL_FEEDRATE;
+  const float manual_feedrate[] = MANUAL_FEEDRATE;
   static void lcd_main_menu();
   static void lcd_tune_menu();
   static void lcd_prepare_menu();