libbpf: Add enum64 support for bpf linking

Add BTF_KIND_ENUM64 support for bpf linking, which is
very similar to BTF_KIND_ENUM.

Acked-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/r/20220607062642.3721494-1-yhs@fb.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>