libbpf: Move feature detection code into its own file

It's quite a lot of well isolated code, so it seems like a good
candidate to move it out of libbpf.c to reduce its size.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20240124022127.2379740-24-andrii@kernel.org