build: move libobj-y variable to nested Makefile.objs

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/target-m68k/Makefile.objs b/target-m68k/Makefile.objs
index e73a5ba..553691d 100644
--- a/target-m68k/Makefile.objs
+++ b/target-m68k/Makefile.objs
@@ -1 +1,4 @@
 obj-y += m68k-semi.o
+obj-y += op_helper.o helper.o cpu.o
+
+$(obj)/op_helper.o: QEMU_CFLAGS += $(HELPER_CFLAGS)