RFR (trivial): 8248906: runtime/Thread/ThreadObjAccessAtExit.java fails due to OutOfMemoryErrors

[David Holmes]

Bug: https://bugs.openjdk.java.net/browse/JDK-8248906
webrev: http://cr.openjdk.java.net/~dholmes/8248906/webrev/

This rare OOME with ZGC doesn't reproduce readily and has not reappeared 
in the mainline CI. However it has failed twice in Panama testing for 
the same reasons. A personal test job also showed that ZGC plus JFR 
fails in a more extreme way (VM initialization failure) and that failure 
is completely reproducible. Bumping the minimum heap to 11M fixed that 
issue so that is what I will commit for this fix.

Testing via tiers 1-3 plus manual on Linux.

Thanks,
David