libbpf: Ensure new BTF objects inherit input endianness

New split BTF needs to preserve base's endianness. Similarly, when
creating a distilled BTF, we need to preserve original endianness.

Fix by updating libbpf's btf__distill_base() and btf_new_empty() to retain
the byte order of any source BTF objects when creating new ones.

Fixes: ba45136 ("libbpf: Implement basic split BTF support")
Fixes: 58e185a ("libbpf: Add btf__distill_base() creating split BTF with distilled base BTF")
Reported-by: Song Liu <song@kernel.org>
Reported-by: Eduard Zingerman <eddyz87@gmail.com>
Suggested-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Tony Ambardar <tony.ambardar@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Tested-by: Alan Maguire <alan.maguire@oracle.com>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/bpf/6358db36c5f68b07873a0a5be2d062b1af5ea5f8.camel@gmail.com/
Link: https://lore.kernel.org/bpf/20240830095150.278881-1-tony.ambardar@gmail.com

Author: guidosarducci

tools/lib/bpf/btf.c

@@ -996,6 +996,7 @@ static struct btf *btf_new_empty(struct btf *base_btf)
 		btf->base_btf = base_btf;
 		btf->start_id = btf__type_cnt(base_btf);
 		btf->start_str_off = base_btf->hdr->str_len;
+		btf->swapped_endian = base_btf->swapped_endian;
 	}
 
 	/* +1 for empty string at offset 0 */
@@ -5394,6 +5395,9 @@ int btf__distill_base(const struct btf *src_btf, struct btf **new_base_btf,
 	new_base = btf__new_empty();
 	if (!new_base)
 		return libbpf_err(-ENOMEM);
+
+	btf__set_endianness(new_base, btf__endianness(src_btf));
+
 	dist.id_map = calloc(n, sizeof(*dist.id_map));
 	if (!dist.id_map) {
 		err = -ENOMEM;