libbpf: Make use of BTF field iterator in BTF handling code

Use new BTF field iterator logic to replace all the callback-based
visitor calls. There is still a .BTF.ext callback-based visitor APIs
that should be converted, which will happens as a follow up.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Tested-by: Alan Maguire <alan.maguire@oracle.com>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/bpf/20240605001629.4061937-4-andrii@kernel.org