[miri] Throw UB if target size and data size don't match

[miri] Throw UB if target size and data size don't match

If an extern C function is defined as

```
extern "C" {
    fn malloc(size: u32) -> *mut std::ffi::c_void;
}
```

on a 64-bit machine(ie. pointer sizes don't match), throw an undefined
behaviour.