bpf: iterators: Build and use lightweight bootstrap version of bpftool

kernel/bpf/preload/iterators use bpftool for vmlinux.h, skeleton, and
static linking only. So we can use lightweight bootstrap version of
bpftool to handle these, and it will be faster.

Suggested-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Pu Lehui <pulehui@huawei.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220714024612.944071-4-pulehui@huawei.com