std_detect is_x86_feature_detected macro should reject unstable features if they are not enabled

[gnzlbg]

E.g. "sse4a", we should check all others as well.

[gnzlbg]

So we can't easily fix this because is_x86_feature_detected has an #[allow_rustc_unstable] attribute to be able to use the stdsimd_internal Feature enum.

I was able to get some compile-fail tests to run by using rustdoc compile_fail feature, removing #[allow_rustc_unstable], and marking, e.g., the mmx enum variant of the Feature enum with the #[unstable(mmx_target_feature, ...)] attribute.

I talked with @oli-obk on Discord and the better solution here is to require #[allow_rustc_unstable] to specify which features it allows explicitly, e.g., #[allow_rustc_unstable(stdsimd_internal)]. That would allow that particular feature, but if the mmx variant of the enum is used without the feature(mmx_target_feature) compilation would fail.