Word tearing problem with _last_sweep

With the change for JDK-8235573, the static variable _last_sweep in objectSampler.cpp can be modified and read concurrently. For 64 bit platforms, this isn't a problem but for 32 bit platforms that support concurrent weak reference processing, this could word tear.

See [~kbarrett] reply for in depth analysis.
https://mail.openjdk.java.net/pipermail/hotspot-dev/2020-August/042614.html