`safe` keyword is not feature-gated

In #126749, @ehuss reports:

Another issue appears that `safe` is not feature-gated. The following seems to build without a feature gate:

```rust
safe fn foo() {}
```

_Originally posted by @ehuss in https://github.com/rust-lang/rust/issues/126749#issuecomment-2181175095_

Tracking:

- https://github.com/rust-lang/rust/issues/123743