RFR (S) 8197844: JVMTI GetLoadedClasses should use the Access API

[coleen.phillimore at oracle.com]

Summary: Make sure the holder of a class loader is accessed during 
iteration of CLDG

This is where we should have put the GC barrier.  This can be cleaned 
somewhat when we have a WeakHandle holder in the ClassLoaderData, then 
the code in ensure_loader_alive() becomes _holder.resolve();

Tested with tier1-5.

open webrev at http://cr.openjdk.java.net/~coleenp/8197844.01/webrev
bug link https://bugs.openjdk.java.net/browse/JDK-8197844

Thanks,
Coleen