Rollup merge of #147049 - vexide:vex-std, r=workingjubilee

std: fix warning in VEXos stdio module

Fixes building `std` on the `armv7a-vex-v5` target due to an unnecessarily mutable argument in `Stdin`.

This was a stupid oversight on my part towards the end of rust-lang/rust#145973's review process. Missed a warning and had a bad bootstrap config that didn't tell me about it when testing changes.

Author: matthiaskrgr