[15] RFR 8245880: Shenandoah: should not mark all code roots if class unloading is enabled

[Zhengyu Gu]

Currently, concurrent code root scanning marks all code cache, no matter 
if class unloading is enabled, which is wrong.

When class unloading is enabled, only on-stack code roots should be marked.


Bug: https://bugs.openjdk.java.net/browse/JDK-8245880
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8245880/webrev.00/

Test:
   hotspot_gc_shenandoah
   tier1 with Shenandoah


Thanks,

-Zhengyu