selftests/bpf: add CO-RE relocs enum/ptr/func_proto tests

Test CO-RE relocation handling of ints, enums, pointers, func protos, etc.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>