Need way for custom linker step to be able to log MSBuild warnings

Context: https://github.com/xamarin/xamarin-android/pull/6423

xamarin/xamarin-android needs to be able to do something like:

```csharp
Context.LogMessage (string.Format ("warning XA2000: " + Resources.XA2000, assembly));
```

Unfortunately, the linker doesn't log this message unless you pass `--verbose`.

So it currently doesn't appear there is a good way to log warnings. We don't want `--verbose`, because it would log a lot of extra stuff.

Could messages with `warning` or `error` always log regardless of `--verbose`? A new API would also work for us.