selftests/bpf: Bring back c++ include/link test

Commit 5c26f9a7 ("libbpf: Don't use cxx to test_libpf target")
converted existing c++ test to c. We still want to include and
link against libbpf from c++ code, so reinstate this test back,
this time in a form of a selftest with a clear comment about
its purpose.

v2:
* -lelf -> $(LDLIBS) (Andrii Nakryiko)

Fixes: 5c26f9a7 ("libbpf: Don't use cxx to test_libpf target")
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20191202215931.248178-1-sdf@google.com