[E0658] Use of unstable feature

[MahadMuhammad]

E0658 Use of Unstable feature

• Added Errorcode Support for unstable features.

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Code Tested:

extern "rust-intrinsic" { // { dg-error "intrinsics are subject to change." "" { target *-*-* }  }
    fn foo(); // { dg-error "unrecognized intrinsic function: .foo." }
}

fn main() {
    unsafe {
        foo(); // { dg-error "unrecognized intrinsic function: .foo." }
    }
}

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Output:

/gccrs/gcc/testsuite/rust/compile/feature_extern_types.rs:2:5: error: extern types are experimental. see issue 43467 <https://github.com/rust-lang/rust/issues/43467> for more information. add `#![feature(extern_types)]` to the crate attributes to enable. [E0658]
compiler exited with status 1

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gcc/rust/ChangeLog:

* checks/errors/rust-feature-gate.cc (FeatureGate::gate): called error function.

gcc/testsuite/ChangeLog:

* rust/compile/changed_intrinsics.rs: New test.

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[philberty]

LGTM