RFR(XS): 8251179: Word tearing problem with _last_sweep

[Markus Gronlund]

Greetings,

This is a follow-up to 8235573: Move JFR ObjectSample oop into OopStorage.

As of 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. This change adds atomic accesses for said variable.

Bug: https://bugs.openjdk.java.net/browse/JDK-8251179 
Webrev: http://cr.openjdk.java.net/~mgronlun/8251179/webrev00/ 
Testing: jdk_jfr

Thank you
Markus