The ISR looks like:

```rust
#[no_mangle]
unsafe extern "C" fn _irq_handler() {
    hprintln!("> IRQ");
    while let Some(int_id) = GicV3::get_and_acknowledge_interrupt() {
        hprintln!("- IRQ handle {:?}", int_id);
        GicV3::end_interrupt(int_id);
    }
    hprintln!("< IRQ");
}
```

I suppose I could write a macro to type-check the function signature, but I haven't.