selftests/bpf: put test_stub.o into $(OUTPUT)

Add a rule to put test_stub.o in $(OUTPUT) and change the references to
it accordingly. This prevents test_stub.o from being created in the
source directory.
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

tools/testing/selftests/bpf/Makefile

@@ -83,13 +83,16 @@ all: $(TEST_CUSTOM_PROGS)
 $(OUTPUT)/urandom_read: $(OUTPUT)/%: %.c
 	$(CC) -o $@ $< -Wl,--build-id
 
+$(OUTPUT)/test_stub.o: test_stub.c
+	$(CC) $(TEST_PROGS_CFLAGS) $(CFLAGS) -c -o $@ $<
+
 $(OUTPUT)/test_maps: map_tests/*.c
 
 BPFOBJ := $(OUTPUT)/libbpf.a
 
-$(TEST_GEN_PROGS): test_stub.o $(BPFOBJ)
+$(TEST_GEN_PROGS): $(OUTPUT)/test_stub.o $(BPFOBJ)
 
-$(TEST_GEN_PROGS_EXTENDED): test_stub.o $(OUTPUT)/libbpf.a
+$(TEST_GEN_PROGS_EXTENDED): $(OUTPUT)/test_stub.o $(OUTPUT)/libbpf.a
 
 $(OUTPUT)/test_dev_cgroup: cgroup_helpers.c
 $(OUTPUT)/test_skb_cgroup_id_user: cgroup_helpers.c