selftests/bpf: Rewrite recvmsg{4,6} asm progs to c in test_sock_addr

I'll extend them in the next patch. It's easier to work with C
than with asm.

Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20210127232853.3753823-4-sdf@google.com