Rollup merge of #158523 - JonathanBrouwer:repr_simd, r=mejrs

Fix feature gate for `repr(simd)`

The following code currently compiles on stable rust:
```rust
#[repr(simd)]
println!()
```

This has the following behavior:
- 1.90 and earlier give an "unused attribute warning"
- 1.91 ..= beta give no diagnostic output
- nightly gives the following:
```
warning: `#[repr(simd)]` attribute cannot be used on macro calls
 --> src/main.rs:4:5
  |
4 |     #[repr(simd)]
  |     ^^^^^^^^^^^^^
  |
  = help: `#[repr(simd)]` can only be applied to structs
  = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
  = note: `#[warn(unused_attributes)]` (part of `#[warn(unused)]`) on by default
```

This PR changes this to a feature gate **error**.
r? @mejrs