selftests/bpf: Add a BTF verification test for kflagged type_tag

Add a BTF verification test case for a type_tag with a kflag set.
Type tags with a kflag are now valid.

Add BTF_DECL_ATTR_ENC and BTF_TYPE_ATTR_ENC test helper macros,
corresponding to *_TAG_ENC.

Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20250130201239.1429648-7-ihor.solodrai@linux.dev