RFR(s): 8213560: gtests might hang

[Robbin Ehn]

Hi all, please review.

The VM thread might interfere with the gtest, since the gtest don't respect
safepoint but needs e.g. Threads_lock. E.g. if the VM thread do a safepoint the
gtests can't create test JavaThreads and we deadlock.

To handle this the VMThreadBlocker is used block the VM thread in a
non-safepoint VM operation. The test is signaled to continue as we queue up the
blocking VM op, but leaves a window of opportunity for a real safepoint to slip
in and we deadlock.

By letting the test wait until VMThreadBlocker's VM_StopSafepoint op is actually 
called by VM thread we eliminate that window.

Webrev: http://cr.openjdk.java.net/~rehn/8213560/webrev/index.html
Cr: https://bugs.openjdk.java.net/browse/JDK-8213560

Tested with 10 iterations of gtest on release and fastdebug builds on all our
platforms (80 iterations total).

Thanks, Robbin