docs: do not inline x86 instrinsics into x86-64

[notriddle]

This reduces the size of the core/arch HTML from about 187M to 132M.

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @Amanieu (or someone else) some time within the next two weeks.

[Amanieu]

While this does reduce the size, it actually makes the documentation incorrect: std::arch::x86 can't be accessed from x86-64: you need to use std::arch::x86_64 instead.

[notriddle]

I can remove the incorrect cfg(any()) part, but is the change to an explicitly-visible glob import acceptable?