RFR (M) 8201556: Disallow reading oops in ClassLoaderData if unloading

[coleen.phillimore at oracle.com]

Summary: Move class_loader oop to an OopHandle, and assert that holder 
is alive when getting class_loader.

open webrev at http://cr.openjdk.java.net/~coleenp/8201556.01/webrev
bug link https://bugs.openjdk.java.net/browse/JDK-8201556

Tested with mach5 tiers 1-7.

Thanks,
Coleen