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>