Support cross-compiling for iOS (armv7)

iOS guarantees that that the arguments are UTF8?

Even so, it seems that using [`.as_bytes`](http://doc.rust-lang.org/master/std/c_str/struct.CString.html#method.as_bytes) directly would be nicer

```rust
res.push(Vec::from_slice(s.as_bytes()))
```

(no `match`)