Changes to `u128`/`i128` layout in 1.77 and 1.78

Submitted ⁨⁨7⁩ ⁨months⁩ ago⁩ by ⁨reedbot⁩ to ⁨rust⁩

https://blog.rust-lang.org/2024/03/30/i128-layout-update.html