Cherry-picks from jdk/jdk to shenandoah/jdk

[Zhengyu Gu]

Hi,

I would like to cherry pick following changes from jdk/jdk to 
shenandoah/jdk, as they are needed for x86_32 nmethod barrier and 
concurrent class unloading work.

Cherry-pick 8231250: Shenandoah: Traversal GC should keep alive weak 
load from heap
Webrev: http://cr.openjdk.java.net/~zgu/shenandoah/JDK-8231250/webrev.00/

Cherry-pick 8231410: Shenandoah: clone barrier should use base pointer
Cherry-pick 8231447: Shenandoah: Compilation-time regression after
                      JDK-8231086
Cherry-pick 8231499: Shenandoah: compiler/arraycopy
                      /TestDefaultMethodArrayCloneDeoptC2 fails
http://cr.openjdk.java.net/~zgu/shenandoah/JDK-8231499/webrev.00/

Test:
   hotspot_gc_shenandoah Linux x86_64 and x86_32

Thanks,

-Zhengyu