RFR: Cherry-pick: 8230401: ClassLoaderData::_keep_alive is read with wrong type in c2i entry barrier

[Zhengyu Gu]

I would like to cherry pick this patch to shenandoah/jdk. It is needed 
for x86_32 nmethod barrier work.

http://cr.openjdk.java.net/~zgu/shenandoah/JDK-8230401_CLD-keep_alive/webrev.00/

Test:
   hotspot_gc_shenandoah x86_64 (fastdebug and release)


Thanks,

-Zhengyu