Sync portable-simd for bitmasks &c.

In the ideal case, where everything works easily and nothing has to be rearranged, it is as simple as:
- `git subtree pull -P library/portable-simd https://github.com/rust-lang/portable-simd - ${branch}`
- write the commit message
- `python x.py test --stage 1` to make sure it runs
- `git push` to your PR-to-rustc branch

If anything borks up this flow, you can fix it with sufficient git wizardry but you are usually better off going back to the source, fixing it, and starting over, before you open the PR.

r? `@calebzulawski`