libbpf: Move CO-RE types into relo_core.h.

In order to make a clean split of CO-RE logic move its types
into independent header file.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210721000822.40958-4-alexei.starovoitov@gmail.com