RFR(T) 8222086: CodeCache::UnloadingScope needs to preserve and restore previous IsUnloadingBehavior

[Zhengyu Gu]

Shenandoah can (will) do concurrent class unloading, but also unloads 
classes at pauses, e.g. during full gc.

Currently, CodeCache::UnloadingScope's destructor resets 
IsUnloadingBehaviour that is setup for concurrent class unloading, which 
results fatal crash during next concurrent unloading cycle.

Bug: https://bugs.openjdk.java.net/browse/JDK-8222086
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8222086/webrev.00/


Test:
   hotspot_gc on Linux x64 (fastdebug and release)
   Submit tests.

Thanks,

-Zhengyu