Makefile: build an intermediate object

Pretty often it happens that during trials we pick wrong external
functions (e.g., "printf") and the final link claims horribly about
dozens of undefined symbols.  By having an intermediate wrc.o we can
easily check which are the symbols we really leave undefined for libc
and libgcc to fill, before they resolve them by asking for something
else.

Signed-off-by: Alessandro Rubini <rubini@gnudd.com>

Makefile

@@ -7,6 +7,7 @@ BOARD = spec
 CROSS_COMPILE ?= lm32-elf-
 
 CC =		$(CROSS_COMPILE)gcc
+LD =		$(CROSS_COMPILE)ld
 OBJDUMP =	$(CROSS_COMPILE)objdump
 OBJCOPY =	$(CROSS_COMPILE)objcopy
 SIZE =		$(CROSS_COMPILE)size
@@ -93,12 +94,15 @@ all: tools $(OUTPUT).ram $(OUTPUT).vhd
 .PRECIOUS: %.elf %.bin
 .PHONY: all tools clean gitmodules
 
-$(OUTPUT).elf: $(LDS) silentoldconfig gitmodules $(OBJS)
+$(OUTPUT).elf: $(LDS) silentoldconfig gitmodules $(OUTPUT).o
 	$(CC) $(CFLAGS) -DGIT_REVISION=\"$(REVISION)\" -c revision.c
-	${CC} -o $@ revision.o $(OBJS) $(LDFLAGS)
+	${CC} -o $@ revision.o $(OUTPUT).o $(LDFLAGS)
 	${OBJDUMP} -d $(OUTPUT).elf > $(OUTPUT)_disasm.S
 	$(SIZE) $@
 
+$(OUTPUT).o: $(OBJS)
+	$(LD) --gc-sections -e _start -r $(OBJS) -o $@
+
 %.bin: %.elf
 	${OBJCOPY} -O binary $^ $@