Implement tcx BPF link support for libbpf.

The bpf_program__attach_fd() API has been refactored slightly in order to pass
bpf_link_create_opts pointer as input.

A new bpf_program__attach_tcx() has been added on top of this which allows for
passing all relevant data via extensible struct bpf_tcx_opts.

The program sections tcx/ingress and tcx/egress correspond to the hook locations
for tc ingress and egress, respectively.

For concrete usage examples, see the extensive selftests that have been
developed as part of this series.
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20230719140858.13224-5-daniel@iogearbox.netSigned-off-by: Alexei Starovoitov <ast@kernel.org>