RFR: 8022853: add ability to load uncompressed object and Klass references in a compressed environment to Unsafe

[Christian Thalinger]

https://bugs.openjdk.java.net/browse/JDK-8022853

In some native data structures of the VM oops are stored uncompressed even if compressed oops are turned on, for example Klass::_java_mirror.  Java code which is tightly coupled with the VM should have the ability to read and write such fields.

This additional Unsafe API make the implementation of JEP 243: Java-Level JVM Compiler Interface simpler.

http://cr.openjdk.java.net/~twisti/8022853/hotspot/webrev.01/
http://cr.openjdk.java.net/~twisti/8022853/jdk/webrev.01/