allow_internal_unstable needs a list of features

[hellow554]

https://github.com/rust-lang-nursery/stdsimd/blob/b23541340b5941749e5fbb1930e666bbd1375244/crates/std_detect/src/detect/arch/x86.rs#L82

[00:25:35] warning: allow_internal_unstable expects list of feature names. In the future this will become a hard error. Please use `allow_internal_unstable(foo, bar)` to only allow the `foo` and `bar` features
[00:25:35]   --> /checkout/src/libstd/../stdsimd/crates/std_detect/src/detect/arch/x86.rs:82:1
[00:25:35]    |
[00:25:35] 82 | #[allow_internal_unstable]
[00:25:35]    | ^^^^^^^^^^^^^^^^^^^^^^^^^^

(source)

[gnzlbg]

@hellow554 stdsimd can be updated upstream