[lworld] RFR: Unsafe primitives for values

[Mandy Chung]

Webrev: 
http://cr.openjdk.java.net/%7Emchung/valhalla/webrevs/unsafe/private-buffer.00/ 
This patch adds Unsafe primitives to test if a field is flattened and to 
allow updating a value using putXXX in a private buffered value that is 
in a larval state. The new Unsafe APIs for values include the following: 
boolean Unsafe::isFlattened(Field f);boolean 
Unsafe::isFlattenedArray(Class<?> arrayClass); <V> long 
valueHeaderSize(Class<V> vc); <V> V makePrivateBuffer(V value); <V> V 
finishPrivateBuffer(V value); <V> V getValue(Object o, long offset, 
Class<?> vc); <V> void putValue(Object o, long offset, Class<?> vc, V 
v); makePrivateBuffer creates a value instance whose layout and contents 
is exactly the same as the input value and marks it in larval state and 
finishPrivateBuffer will exit the larval state. In between Unsafe.putXXX 
on the buffer will update the larval value instance. Mandy