fix crash when encountering invokespecial call to abstract method

We must throw an AbstractMethodError when such a call is executed (not
when the call is compiled), so we compile this case as a call to a
thunk which throws such an error.

classpath/java/lang/AbstractMethodError.java

@@ -0,0 +1,21 @@
+/* Copyright (c) 2011, Avian Contributors
+
+   Permission to use, copy, modify, and/or distribute this software
+   for any purpose with or without fee is hereby granted, provided
+   that the above copyright notice and this permission notice appear
+   in all copies.
+
+   There is NO WARRANTY for this software.  See license.txt for
+   details. */
+
+package java.lang;
+
+public class AbstractMethodError extends IncompatibleClassChangeError {
+  public AbstractMethodError() {
+    super();
+  }
+
+  public AbstractMethodError(String message) {
+    super(message);
+  }
+}