selftests/bpf: Rename progs/tag.c to progs/btf_decl_tag.c

Rename progs/tag.c to progs/btf_decl_tag.c so we can introduce
progs/btf_type_tag.c in the next patch.

Also create a subtest for btf_decl_tag in prog_tests/btf_tag.c
so we can introduce btf_type_tag subtest in the next patch.

I also took opportunity to remove the check whether __has_attribute
is defined or not in progs/btf_decl_tag.c since all recent
clangs should already support this macro.

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20211112012641.1507144-1-yhs@fb.com