Our polyfill for __float128 should have 16 byte alignment

[fitzgen]

Right now it is an array of u8 and has 1 byte alignment, I think.

u128 and i128 just landed in rustc, and we should take advantage of them here until Rust has a proper f128 since they should have the alignment restrictions we need.

This will only be available on nightly / unstable for a bit I think.

[pvdrz]

This already happens. I wasn't able to bisect to get the right commit due to not being able to compile old versions of bindgen with the right rustc version