Bootstrapping on completely new platforms

@uweigand brought up that `./prep.sh --bootstrap` does not help for platforms that are not currently supported by Microsoft because no packages exist for those platforms. To work around this, he is manually building a subset of runtime that produces the necessary packages - these are all native bits so they do not require a bootstrap compiler. This is something that we could add to bootstrapping and may even be preferable to the current approach of downloading the Microsoft-built packages.