common.mak: Use CCFLAGS for assembly generation as well

CCFLAGS is equivalent to CPPFLAGS + CFLAGS, and it is already being used
by other make rules like %.i and %.o. Simplifies common.mak.

common.mak

@@ -67,7 +67,7 @@ COMPILE_HOSTC = $(call COMPILE,HOSTCC)
 	$(COMPILE_M)
 
 %.s: %.c
-	$(CC) $(CPPFLAGS) $(CFLAGS) -S -o $@ $<
+	$(CC) $(CCFLAGS) -S -o $@ $<
 
 %.o: %.S
 	$(COMPILE_S)