py/mkrules.mk: Add MPY_CROSS_FLAGS option to pass flags to mpy-cross.

So that ports can pass their own custom options to mpy-cross.

py/mkrules.mk

@@ -115,7 +115,7 @@ FROZEN_MPY_MPY_FILES := $(addprefix $(BUILD)/frozen_mpy/,$(FROZEN_MPY_PY_FILES:.
 $(BUILD)/frozen_mpy/%.mpy: $(FROZEN_MPY_DIR)/%.py
 	@$(ECHO) "MPY $<"
 	$(Q)$(MKDIR) -p $(dir $@)
-	$(Q)$(MPY_CROSS) -o $@ -s $(^:$(FROZEN_MPY_DIR)/%=%) $^
+	$(Q)$(MPY_CROSS) -o $@ -s $(^:$(FROZEN_MPY_DIR)/%=%) $(MPY_CROSS_FLAGS) $^
 
 # to build frozen_mpy.c from all .mpy files
 $(BUILD)/frozen_mpy.c: $(FROZEN_MPY_MPY_FILES) $(BUILD)/genhdr/qstrdefs.generated.h