pthread_attr_t definition is incorrect, will cause unaligned memory accesses

It's not correct to bind a `union` in Rust as an array of bytes. The alignment of a byte array is always 1.

``` rust
struct Foo {
    xs: [u8, ..64]
}

fn main() {
    println!("{}", std::mem::min_align_of::<Foo>())
}
```

x86 allows unaligned memory accesses, but other architectures are not as forgiving.
