Naked functions do not require unsafety

[RFC](https://github.com/rust-lang/rfcs/blob/master/text/1201-naked-fns.md) states that naked functions must either not contain any “safe” blocks or be unsafe themselves. Currently, code like 

```
#![feature(asm, naked_functions)]

#[naked]
extern "win64" fn naked(x: u32) -> u32 {
    x + 1
}
```

is accepted.
