optionrom: build with discrete CPP and AS steps

Build option ROM .S files with separate preprocessor and
assembler steps because the C compiler could be unsuitable.

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
diff --git a/rules.mak b/rules.mak
index 8448b94..fe0c881 100644
--- a/rules.mak
+++ b/rules.mak
@@ -28,8 +28,11 @@
 	$(call quiet-command,$(LIBTOOL) --mode=compile --quiet --tag=CC $(CC) $(QEMU_INCLUDES) $(QEMU_CFLAGS) $(QEMU_DGFLAGS) $(CFLAGS) -c -o $@ $<,"  lt CC $@")
 endif
 
-%.o: %.S
-	$(call quiet-command,$(CC) $(QEMU_INCLUDES) $(QEMU_CFLAGS) $(QEMU_DGFLAGS) $(CFLAGS) -c -o $@ $<,"  AS    $(TARGET_DIR)$@")
+%.asm: %.S
+	$(call quiet-command,$(CPP) $(QEMU_INCLUDES) $(QEMU_CFLAGS) $(QEMU_DGFLAGS) $(CFLAGS) -o $@ $<,"  CPP   $(TARGET_DIR)$@")
+
+%.o: %.asm
+	$(call quiet-command,$(AS) $(ASFLAGS) -o $@ $<,"  AS    $(TARGET_DIR)$@")
 
 %.o: %.m
 	$(call quiet-command,$(OBJCC) $(QEMU_INCLUDES) $(QEMU_CFLAGS) $(QEMU_DGFLAGS) $(CFLAGS) -c -o $@ $<,"  OBJC  $(TARGET_DIR)$@")