reduce binary size of .hex

still a few bytes bigger than with arduino but nothing major

Marlin/Makefile

 CWARN = -Wall -Wstrict-prototypes
 CTUNING = -funsigned-char -funsigned-bitfields -fpack-struct \
 	-fshort-enums -w -ffunction-sections -fdata-sections \
+	-flto \
 	-DARDUINO=$(ARDUINO_VERSION)
 ifneq ($(HARDWARE_MOTHERBOARD),)
 CTUNING += -DMOTHERBOARD=${HARDWARE_MOTHERBOARD}
 	# Link: create ELF output file from library.
 $(BUILD_DIR)/$(TARGET).elf: $(OBJ) Configuration.h
 	$(Pecho) "  CXX   $@"
-	$P $(CC) $(ALL_CXXFLAGS) -Wl,--gc-sections -o $@ -L. $(OBJ) $(LDFLAGS)
+	$P $(CC) $(ALL_CXXFLAGS) -Wl,--gc-sections,--relax -o $@ -L. $(OBJ) $(LDFLAGS)
 
 $(BUILD_DIR)/%.o: %.c Configuration.h Configuration_adv.h $(MAKEFILE)
 	$(Pecho) "  CC    $<"