libbpf: Open code low level bpf commands.

Open code low level bpf commands used by light skeleton to
be able to avoid full libbpf eventually.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20220131220528.98088-3-alexei.starovoitov@gmail.com