OpenOptions are not checked

The following program does not panic on Hermit, although it should return `EBADF` on `read`:

```rust
use std::fs::File;
use std::io::{Read, Write};

#[cfg(target_os = "hermit")]
use hermit as _;

const BUF_SIZE: usize = 8 * 1024;

fn main() {
let mut file = File::create("/tmp/hello.txt").unwrap();
let mut buf = vec![0; BUF_SIZE];
file.read(&mut buf).unwrap();
}
```