libbpf: Cleanup the layering between CORE and bpf_program.

CO-RE processing functions don't need to know 'struct bpf_program' details.
Cleanup the layering to eventually be able to move CO-RE logic into a separate file.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210721000822.40958-2-alexei.starovoitov@gmail.com