set InvocationTargetException target

When using the OpenJDK classpath, the target exception and the
Throwable cause are two different fields; we must set them both.

classpath/java/lang/reflect/InvocationTargetException.java

@@ -11,6 +11,8 @@
 package java.lang.reflect;
 
 public class InvocationTargetException extends Exception {
+  private Throwable target; // for compatibility with OpenJDK
+
   public InvocationTargetException(Throwable targetException, String message) {
     super(message, targetException);
   }

src/avian/classpath-common.h

@@ -561,6 +561,9 @@ invoke(Thread* t, object method, object instance, object args)
         object exception = t->exception;
         t->exception = makeThrowable
           (t, Machine::InvocationTargetExceptionType, 0, 0, exception);
+        
+        set(t, t->exception, InvocationTargetExceptionTarget,
+            throwableCause(t, t->exception));
       }
     });