RFR: 8206144: [lworld] Adapt and add C2 intrinsics for value types Unsafe API

[Roland Westrelin]

http://cr.openjdk.java.net/~roland/8206144/webrev.00/

This change:

- improves robustness of getX intrinsics when reading from a value type
  (mismatched accesses should now be correctly handled).

- implements intrinsics for getValue and putValue

- implements intrinsics for makePrivateBuffer/finishPrivateBuffer. I
  added a larval bit to TypeValueType. I'm not entirely convince it's
  the right approach but that could be revisited later.

Roland.