maintain a table to look up methods called via JNI

This simplifies the JNI implementation for looking up methods.  It also
fixes a bug where an applications calls GetStaticMethodID with class A
and then calls CallStatic<Type>Method with class B which extends A.  The
old code would look in the wrong method table and thus call the wrong
method.

classpath/java/lang/reflect/Method.java

@@ -17,6 +17,7 @@ public class Method<T> extends AccessibleObject implements Member {
   private byte parameterFootprint;
   private short flags;
   private short offset;
+  private int nativeID;
   private byte[] name;
   private byte[] spec;
   private Class<T> class_;