ObjectMonitor::_contentions field should not be 'jint'

[~coleenp] mentioned this RFE during the JDK-8253064 review:

 src/hotspot/share/runtime/objectMonitor.hpp
@@ -179,7 +171,7 @@ class ObjectMonitor {
  jint _contentions; // Number of active contentions in enter(). It is used by is_busy()

Future RFE - can we replace jint with int32_t or even int or some C++ types. We're trying not to have Java types leak into runtime code since this doesn't directly interface with Java.