Make jnimarshalmethod-gen not depend on creating JVM

We use the `JniEnvironment.Runtime.MarshalMemberBuilder` in `jnimarshalmethod-gen` and that means we need to create JVM first. That is currently [broken on Windows](https://github.com/xamarin/java.interop/issues/713) and hopefully not neccessary. 

It would make the `jnimarshalmethod-gen` initialization faster and simplify things. We could remove the `--jvm` option as well.
