Can't stub out a P/Invoke from an xml substitution file

It doesn't look like I can use an xml substitution file to stub out a P/Invoke, which while not a big issue, IMHO it would make sense to make it work.

Example xml:

```xml

```

Example cs:

```cs
namespace ObjCRuntime;
class Runtime {
[DllImport ("whatever")]
static extern int get_runtime_arch ();
}
```

Workaround:

```xml

```

```cs
namespace ObjCRuntime;
class Runtime {
[DllImport ("whatever")]
static extern int get_runtime_arch ();
static int GetRuntimeArch ()
{
return get_runtime_arch ();
}
}
```

and then the linker links away the P/Invoke, since it's not used anymore (after the substitution).