RFR: JDK-8205549 JDK-8205698 Support of flattened values in Unsafe

[Frederic Parain]

Please review this changeset which adds support for flattened values to the Unsafe API.

http://cr.openjdk.java.net/~fparain/Unsafe/webrev.02/index.html

This changeset addresses the following issues:
  - reading from /writing to a flattenable/flattened field with Unsafe
  - null-check when writing to a flattenable field
  - initialization barrier when reading an uninitialized static value field with Unsafe
  - enforcing values immutability even when using Unsafe
  - reading from /writing to a value array with Unsafe
  - null-check when writing to a value array with Unsafe

Thanks,

Fred