`#![feature(stdsimd)]` was split up and removed in 1.78, so stop emitting it when we upgrade past 1.65

[kkysen]

We still emit #![feature(stdsimd)] in a lot of places because we're still pinned to an old nightly on 1.65, but in 1.78 (rust-lang/rust#117372), it was split into multiple features. So when we upgrade to stable/a newer nightly for the transpiler output, we need to handle this.