Fix: enable stdarch_loongarch feature for stdext crate

[MutsukiC]

Fix

error[E0658]: use of unstable library feature `stdarch_loongarch`
...
    = note: see issue #117427 <https://github.com/rust-lang/rust/issues/117427> for more information
    = help: add `#![feature(stdarch_loongarch)]` to the crate attributes to enable
    = note: this compiler was built on 2026-04-28; consider upgrading it if it is out of date

when building edit on loongarch