Request for Review: minor VM class loading change

[Karen Kinnear]

Please review a minor change for the VM to handle a non-null base module loader. I've also
added a new (potentially temporary) API JVM_GetModuleLoader to allow us to incrementally clean up logic that assumes
that JVM_GetClassLoader returns null for the bootclasspath.

Within the VM, we will continue to use null to represent module classes loaded by the vm.

http://cr.openjdk.java.net/~acorn/non-null-module-loader/webrev/

thanks,
Karen