tools/bpftool: Use libbpf's btf__parse() API for parsing BTF from file

Use generic libbpf API to parse BTF data from file, instead of re-implementing
it in bpftool.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20200802013219.864880-3-andriin@fb.com