btf__add_union
err = btf__add_union(btf_new, name, type->size);
LIBBPF_API int btf__add_union(struct btf *btf, const char *name, __u32 sz);
btf__add_union(btf1, "u1", 12); /* [6] union u1 { */
btf__add_union(btf1, "", 4); /* [7] union { */
btf__add_union(btf1, "u1", 4); /* [17] union u1 { */
btf__add_union(btf, "u1", 12); /* [6] union u1 { */
btf__add_union(btf3, "u1", 16); /* [5] union u1 { */
id = btf__add_union(btf, "u1", 8);