build: sort objects to remove duplicates for link

Avoid duplicate object files during the link.  There are legitimate
cases where a link command-line would include duplicate object files
because two independent subsystems both depend on common infrastructure.

Use GNU make's $(sort) function to remove duplicate object files from
the link command-line.
Signed-off-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

rules.mak

@@ -31,7 +31,7 @@ endif
 %.o: %.m
 	$(call quiet-command,$(CC) $(QEMU_INCLUDES) $(QEMU_CFLAGS) $(QEMU_DGFLAGS) $(CFLAGS) -c -o $@ $<,"  OBJC  $(TARGET_DIR)$@")
 
-LINK = $(call quiet-command,$(CC) $(QEMU_CFLAGS) $(CFLAGS) $(LDFLAGS) -o $@ $(1) $(LIBS),"  LINK  $(TARGET_DIR)$@")
+LINK = $(call quiet-command,$(CC) $(QEMU_CFLAGS) $(CFLAGS) $(LDFLAGS) -o $@ $(sort $(1)) $(LIBS),"  LINK  $(TARGET_DIR)$@")
 
 %$(EXESUF): %.o
 	$(call LINK,$^)