Consider adding a `//@ needs-std` directive for target std support

[jieyouxu]

Hmm, that directive is never actually used, and also does not solve the issue for that target. It is a known target, just not one that has std (just core I assume). None of the directives in the list seem to really capture this? Maybe needs-threads is a decent proxy but that's not ideal.

-- #141856 (comment)

and

Yeah, a proper fix is sth like //@ needs-std, but for this PR, //@ ignore-none is a proxy for ignoring the *-unknown targets. I suppose //@ ignore-nvptx is better tho.

Originally posted by @jieyouxu in #141856 (comment)