libbpf: Mark BPF subprogs with hidden visibility as static for BPF verifier

Define __hidden helper macro in bpf_helpers.h, which is a short-hand for
__attribute__((visibility("hidden"))). Add libbpf support to mark BPF
subprograms marked with __hidden as static in BTF information to enforce BPF
verifier's static function validation algorithm, which takes more information
(caller's context) into account during a subprogram validation.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20210423181348.1801389-5-andrii@kernel.org

src/btf.c

@@ -1605,11 +1605,6 @@ static void *btf_add_type_mem(struct btf *btf, size_t add_sz)
 			      btf->hdr->type_len, UINT_MAX, add_sz);
 }
 
-static __u32 btf_type_info(int kind, int vlen, int kflag)
-{
-	return (kflag << 31) | (kind << 24) | vlen;
-}
-
 static void btf_type_inc_vlen(struct btf_type *t)
 {
 	t->info = btf_type_info(btf_kind(t), btf_vlen(t) + 1, btf_kflag(t));