#[target_feature="±<feat>"] is not properly checked against whitelist

[nagisa]

#[cfg(target_feature="abcd")] was landed with a strong requirement to have a whitelist of LLVM features that are exposed this way, mostly due to compatibility concerns with LLVM.

#[target_feature="±abcd"], however does not check against the whitelist in question and allows everything through. Seems like a potential compat footgun?

[est31]

cc @BurntSushi

[BurntSushi]

It was landed to make it possible to experiment with SIMD support in Rust. I would expect this question to be resolved in the stabilization effort. (But yes, I agree, it does certainly need to be checked.)

[alexcrichton]

This was fixed for #44839