py/mkrules.mk: Use $(CPP) not $(CC) -E for preprocessor rule.

py/mkrules.mk

@@ -53,7 +53,7 @@ vpath %.c . $(TOP) $(USER_C_MODULES)
 
 $(BUILD)/%.pp: %.c
 	$(ECHO) "PreProcess $<"
-	$(Q)$(CC) $(CFLAGS) -E -Wp,-C,-dD,-dI -o $@ $<
+	$(Q)$(CPP) $(CFLAGS) -Wp,-C,-dD,-dI -o $@ $<
 
 # The following rule uses | to create an order only prerequisite. Order only
 # prerequisites only get built if they don't exist. They don't cause timestamp