selftests/bpf: Rewrite test_sock_addr bind bpf into C

I'm planning to extend it in the next patches. It's much easier to
work with C than BPF assembly.

Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20201202172516.3483656-2-sdf@google.com