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 <[email protected]>
Signed-off-by: Alexei Starovoitov <[email protected]>
Acked-by: Andrii Nakryiko <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]

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