selftests/bpf: Build urandom_read with LDFLAGS and LDLIBS

During cross-compilation, it was discovered that LDFLAGS and
LDLIBS were not being used while building binaries, leading
to defaults which were not necessarily correct.

OpenEmbedded reported this kind of problem:

  ERROR: QA Issue: No GNU_HASH in the ELF binary [...], didn't pass LDFLAGS?
Signed-off-by: Daniel Díaz <daniel.diaz@linaro.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: John Fastabend <john.fastabend@gmail.com>

tools/testing/selftests/bpf/Makefile

@@ -122,7 +122,7 @@ $(OUTPUT)/%:%.c
 
 $(OUTPUT)/urandom_read: urandom_read.c
 	$(call msg,BINARY,,$@)
-	$(CC) -o $@ $< -Wl,--build-id
+	$(CC) $(LDFLAGS) -o $@ $< $(LDLIBS) -Wl,--build-id
 
 $(OUTPUT)/test_stub.o: test_stub.c $(BPFOBJ)
 	$(call msg,CC,,$@)