preliminary support for using OpenJDK's class library

Whereas the GNU Classpath port used the strategy of patching Classpath
with core classes from Avian so as to minimize changes to the VM, this
port uses the opposite strategy: abstract and isolate
classpath-specific features in the VM similar to how we abstract away
platform-specific features in system.h.  This allows us to use an
unmodified copy of OpenJDK's class library, including its core classes
and augmented by a few VM-specific classes in the "avian" package.

classpath/java/lang/reflect/Field.java

@@ -204,7 +204,7 @@ public class Field<T> extends AccessibleObject {
       } else {
         throw new IllegalArgumentException
           ("needed " + getType() + ", got "
-           + Class.getName(Class.vmClass(target)) +
+           + Class.getName(SystemClassLoader.vmClass(target)) +
            " when setting " + Class.getName(vmField.class_) + "." + getName());
       }
       break;