RFR(s): 8037112: gc/g1/TestHumongousAllocInitialMark.java caused SIGSEGV

[Per Liden]

Hi,

Could I please have this bugfix reviewed.

Summary: G1's ConcurrentMarkThread tries to use the gclog_or_tty when 
the JVM is about to exit and the gclog_or_tty instance has been 
destroyed. To prevent this, the ConcurrentMarkThread needs to join the 
SuspendibleThreadSet before accessing gclog_or_tty.

Bug: https://bugs.openjdk.java.net/browse/JDK-8037112
Webrev: http://cr.openjdk.java.net/~pliden/8037112/webrev.0/

Thanks!
/Per

Btw, I created a separate RFE, JDK-8039147, to do a little bit of 
cleanup of the SuspendibleThreadSet and how it's used. Will send that 
out as separate review request shortly.