Improve CDS support for custom loaders

In JDK 12, there's experimental support for custom loader in CDS. However, there are limitations:

1. if two or more custom loaders want to load a class named "A", only one of those loaders can actually succeed in loading an archived class.

2. the classlist needs to be crafted (manually, or by a script that parses the output of -Xlog:class+load=debug). We should make it possible to include information for custom loaders automatically with -XX:DumpLoadedClassList.

(TBD - the actual proposal will be discussed on hotspot-runtime-dev@openjdk.java.net).