selftests/bpf: Add BTF_KIND_DECL_TAG typedef example in tag.c

Change value type in progs/tag.c to a typedef with a btf_decl_tag.
With `bpftool btf dump file tag.o`, we have
  ...
  [14] TYPEDEF 'value_t' type_id=17
  [15] DECL_TAG 'tag1' type_id=14 component_idx=-1
  [16] DECL_TAG 'tag2' type_id=14 component_idx=-1
  [17] STRUCT '(anon)' size=8 vlen=2
        'a' type_id=2 bits_offset=0
        'b' type_id=2 bits_offset=32
  ...

The btf_tag selftest also succeeded:
  $ ./test_progs -t tag
    #21 btf_tag:OK
    Summary: 1/0 PASSED, 0 SKIPPED, 0 FAILED
Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20211021195643.4020315-1-yhs@fb.com

tools/testing/selftests/bpf/progs/tag.c

@@ -24,18 +24,23 @@ struct key_t {
 	int c;
 } __tag1 __tag2;
 
+typedef struct {
+	int a;
+	int b;
+} value_t __tag1 __tag2;
+
 struct {
 	__uint(type, BPF_MAP_TYPE_HASH);
 	__uint(max_entries, 3);
 	__type(key, struct key_t);
-	__type(value, __u64);
+	__type(value, value_t);
 } hashmap1 SEC(".maps");
 
 
 static __noinline int foo(int x __tag1 __tag2) __tag1 __tag2
 {
 	struct key_t key;
-	__u64 val = 1;
+	value_t val = {};
 
 	key.a = key.b = key.c = x;
 	bpf_map_update_elem(&hashmap1, &key, &val, 0);