bpftool: add C output format option to btf dump subcommand

Utilize new libbpf's btf_dump API to emit BTF as a C definitions.

Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>