libbpf: Move few APIs from 0.4 to 0.5 version

Official libbpf 0.4 release doesn't include three APIs that were tentatively
put into 0.4 section. Fix libbpf.map and move these three APIs:

  - bpf_map__initial_value;
  - bpf_map_lookup_and_delete_elem_flags;
  - bpf_object__gen_loader.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20210603004026.2698513-2-andrii@kernel.org