[8u20] RFR (S): 8039146: Fix 64-bit store to int JNIHandleBlock::_top

[Vladimir Kozlov]

8u20 backport request. These changes were pushed into jdk9 3 days ago 
and nightly testing shows no problems.

Changes from jdk9 applied to 8u without conflicts.

http://hg.openjdk.java.net/jdk9/hs-comp/hotspot/rev/53471abb7fa0
http://cr.openjdk.java.net/~goetz/webrevs/8039146-intSt/webrev.00/
https://bugs.openjdk.java.net/browse/JDK-8039146

Thanks,
Vladimir