selftests/bpf: Add light skeleton test for kfunc detection.

Add light skeleton test for kfunc detection and denylist it for s390.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20230321203854.3035-5-alexei.starovoitov@gmail.com