JLS changes for VarHandles

From the JEP 193 description:

The following additions will be required to the Java Language Specification:

1. Make reference to the signature-polymorphic access mode methods in the VarHandle class.

2. Allow signature-polymorphic methods to return types other than Object, indicating that the return type is not polymorphic (and would otherwise be declared via a cast at the call site). This makes it easier invoke write-based access methods that return void and invoke compareAndSet that returns a boolean value.