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