Use publicLookup to access generated object fields

Using lookup() appears to return a Lookup object that is stricter
about access restrictions from a parent module to a child
classloader with no module. In this case, the variables in the
reified object classes are to be wired up in accessors. Using the
lookup() version results in an error:

java.lang.IllegalAccessException: symbolic reference class is not accessible: class org.jruby.gen.RubyObject5, from class org.jruby.specialized.RubyObjectSpecializer (module org.jruby.base)

core/src/main/java/org/jruby/specialized/RubyObjectSpecializer.java

@@ -58,7 +58,7 @@
  */
 public class RubyObjectSpecializer {
 
-    public static final MethodHandles.Lookup LOOKUP = MethodHandles.lookup();
+    public static final MethodHandles.Lookup LOOKUP = MethodHandles.publicLookup();
 
     private static ClassAndAllocator getClassForSize(int size) {
         return SPECIALIZED_CLASSES.get(size);