i686 musl cannot panic

e.g. for a simple program like:

```rust
fn main() {
   panic!("hello");
}
```

it runs as:

```
$ rustc foo.rs --target i686-unknown-linux-musl && ./foo
thread 'main' panicked at 'hello', foo.rs:2
note: Run with `RUST_BACKTRACE=1` for a backtrace.
fatal runtime error: failed to initiate panic, error 5
zsh: abort (core dumped)  ./foo
```

cc @japaric 