RFR 8212958: Allow Klass::_subklass and _next_sibling to have unloaded classes

[coleen.phillimore at oracle.com]

Summary: Don't return unloaded klasses. Make sure access is protected by 
Compile_lock.

See bug for more description.  This test has compiler changes since the 
weak klass links in Klass (and implementor in InstanceKlass are used by 
the compiler).  All the compiler jtreg tests passed.

Tested with mach5 tier1-7 and test added.

open webrev at http://cr.openjdk.java.net/~coleenp/8212958.01/webrev
bug link https://bugs.openjdk.java.net/browse/JDK-8212958

Thanks,
Coleen