rustdoc doesn't execute documentation tests for #[target_feature] functions

Example:

```rust
#![feature(target_feature)]

/// Foo
///
/// # Examples
///
/// ```
/// assert!(false);
/// ```
#[target_feature(enable="sse")]
pub unsafe fn foo() {}
```

> $ cargo test

```shell
Doc-tests foo

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
```
