A clobber was surrounded by braces in the `asm` macro.

Erroneous code example:

```compile_fail,E0664
asm!("mov $$0x200, %eax"
     :
     :
     : "{eax}"
    );
```

Considering that this would be a long explanation, we instead recommend you to
take a look at the unstable book:
https://doc.rust-lang.org/unstable-book/language-features/asm.html
