RFR 8224875: Shenandoah: ParallelCleaning code unloading should take lock to protect shared code roots array

[Zhengyu Gu]

The patch changes ShenandoahHeapLock to general purpose spin lock and 
uses it to protect concurrent access to shared array during parallel 
cleaning at safepoints.

Bug: https://bugs.openjdk.java.net/browse/JDK-8224875
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8224875/webrev.00/

Test:
   hotspot_gc_shenandoah (fastdebug and release)

Thanks,

-Zhengyu