An unsupported naked function definition.

Erroneous code example:

```compile_fail,E0787
#[unsafe(naked)]
pub extern "C" fn f() -> u32 {
    42
}
```

The naked function must be defined using a single `naked_asm!` assembly block.

The execution must never fall through past the end of the assembly
code, so it must either return or diverge. The asm block can also
use `att_syntax` and `raw` options, but others options are not allowed.

The asm block must not contain any operands other than `const` and
`sym`.

### Additional information

For more information, please see [RFC 2972].

[RFC 2972]: https://github.com/rust-lang/rfcs/blob/master/text/2972-constrained-naked.md