tools/resolve_btfids: Use libbpf's btf__parse() API

Instead of re-implementing generic BTF parsing logic, use libbpf's API.
Also add .gitignore for resolve_btfids's build artifacts.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20200802013219.864880-4-andriin@fb.com