Add support for wasm64 with simd128

[KonaeAkira]

Even though the memory64 feature has been standardized, the wasm64-unknown-unknown target is currently only a tier 3 target, so it's perfectly understandable if you don't want to merge this PR.

I stumbled upon this while experimenting with building my project with wasm64-unknown-unknown.

[Lokathor]

I don't know that the build_check_only pass uses Nightly at all, so you'll probably have to change it to be a Nightly CI pass.

[KonaeAkira]

Turns out I was looking at the wrong matrix. Thanks :)

[KonaeAkira]

Huh, why is the i686-pc-windows-msvc test suddenly failing ...

[Lokathor]

I think it's the same safe_arch bug that keeps happening. I really have to look in to that.

[Lokathor]

i guess this update is nightly only? since it seems to be enabling a compiler feature.

[KonaeAkira]

Yes, this is for nightly only. wasm64 doesn't really work outside of nightly right now.

[Lokathor]

I think it should always be opt-in for a crate to turn on a nightly compiler feature, such as putting a nightly_wasm64 cargo feature into the crate. and with a comment on the cargo feature declaration that it's not part of our stable semver.