RFR (S) 8175249: VMThread::run fails in VerifyBeforeExit : Universe::verify

[coleen.phillimore at oracle.com]

Summary: protection domain package access cache needs to be walked in 
unloading

Tested with mach5 tier1-5 on linux and windows, and with tests in the 
bug report.

open webrev at http://cr.openjdk.java.net/~coleenp/8175249.01/webrev
bug link https://bugs.openjdk.java.net/browse/JDK-8175249

Thanks,
Coleen