call to __Value methods can't pass fields as arguments

[Roland Westrelin]

http://cr.openjdk.java.net/~roland/valhalla/__valuemethods/webrev.00/

When calling a method that's defined only by __Value (toString etc.),
we can't pass fields, we have to pass a reference (the callee is generic
so has no way to know what fields to expect).

There's a little tweak to some runtime code so someone from runtime
should probably take a look.

Roland.