Not having a recent GCC and not having GCC are different things. There may be architectures that have older GCC versions, but are no longer supported for more current C specs like C11, C23, etc.
I don't believe Rust for Linux use std. I'm not sure how much of Rust for Linux the GCC/Rust effort(s) are able to compile, but if it was "all of it" I'm sure we'd have heard about it.
additionally, I believe the GCC backend is incomplete. the `core` library is able to compile, but rust's `std` cannot be.