Apply @/theoparis' fixes for arm simd

[itsjunetime]

This fixes #571 by applying the changes found in https://github.com/theoparis/pathfinder. I'm not certain why @theoparis did not upstream these themself, but I would like this fix upstreamed, so I'm filing this.

I haven't tested this with versions of nightly before the rust commit that broke this code, so it might break those, but I expect there are more people are using the most recent version of nightly than people using versions that still work with pathfinder_simd.

[theoparis]

I didn't upstream this because I didn't realize these changes would work on stable rust with repr_simd feature-gated - I thought it required nightly rust. Thanks for creating this PR.

[itsjunetime]

It does require nightly, but it seems that pathfinder has their own pf_rustc_nightly cfg to enable compilation that is conditional upon running it under nightly. I didn't realize that at first either, it took me a second of poking to see.