future-incompat: use of SIMD types aren't gated properly

[JohnTitor]

(Splitting from https://github.com/rust-lang/rust/pull/59238/files/8477a04a72a6f8e0adb46be91a2628b07648c200#r266976217)

Consider the following (playground):

use std::arch::x86_64::__m128;

#[allow(improper_ctypes)]
extern "C" {
    fn e(x: __m128);
}

which produces the following error (correctly):

error: use of SIMD type `__m128` in FFI is highly experimental and may result in invalid code
 --> src/lib.rs:5:13
  |
5 |     fn e(x: __m128);
  |             ^^^^^^
  |
  = help: add `#![feature(simd_ffi)]` to the crate attributes to enable

But when we use it on struct (playground):

#[repr(transparent)] pub struct A(__m128);
#[repr(C)] pub struct B(__m128);

it just compiles fine on stable Rust, while we should gate it. We should eventually disallow it through a future incompat lint.

[RalfJung]

With #116558, actually calling that imported function without having the required target features will cause a future-compat lint (eventually error). I'd say that closes this issue.