RFR (L) 8186777: Make Klass::_java_mirror an OopHandle

[coleen.phillimore at oracle.com]

Summary: Add indirection for fetching mirror so that GC doesn't have to 
follow CLD::_klasses

Thank you to Tom Rodriguez for Graal changes and Rickard for the C2 changes.

Ran nightly tests through Mach5 and RBT.   Early performance testing 
showed good performance improvment in GC class loader data processing 
time, but nmethod processing time continues to dominate. Also performace 
testing showed no throughput regression.   I'm rerunning both of these 
performance testing and will post the numbers.

bug link https://bugs.openjdk.java.net/browse/JDK-8186777
local webrev at 
http://oklahoma.us.oracle.com/~cphillim/webrev/8186777.01/webrev

Thanks,
Coleen