Some links in the stable docs point to the unstable docs.

Looks like absolute URLs are used in the overall descriptions [here](https://doc.rust-lang.org/std/arch/index.html) and [here](https://doc.rust-lang.org/core/arch/index.html).

Example in the source code [here](https://github.com/rust-lang-nursery/stdsimd/blob/66267294183beef1349d107443fd4c0103a227ac/stdsimd/mod.rs#L191).