rebooting OpenJDK mobile

Hi, After a long time, it is a good moment now to restart the public work in the OpenJDK Mobile project. In the past, we had a repository with code based on OpenJDK 9 that allows to build the class libraries (including native code) and one or more VM's, for Android and iOS. While this works, we still have the limitation on iOS where dynamic code is not allowed, hence a JIT is not an option. The Zero VM works there (interpreter mode) but is slow. Today, we can use the GraalVM AOT compiler to compile the code at build time. We can link the compiled Java code with native libraries compiled for the target OS, and create executables. Most of the VM functionality is provided by a very small set of files in SubstrateVM (linked to by GraalVM native image) We already did this for iOS, based on Java 11 (see https://github.com/johanvos/openjdk-mobile11/tree/mobile) and JavaFX 13. (see our announcement at https://gluonhq.com/java-on-ios-for-real/) The diff to the upstream code is remarkably small. It is my goal now to use project Skara to create a synchronized fork of the OpenJDK master, and to push the changes required to build the native libs for the core libraries in there. Ultimately, it would be ideal if all required changes can go upstream. At this moment, this is iOS only, but there are no showstoppers to do this for Android as well. The architectures are pretty similar. - Johan