RFR(S): 8194482: Fix SIGSEGV in print_threads_compiling.

[Lindenmaier, Goetz]

Hi,

print_threads_compiling prints the tasks compiler threads are working on.
It gets the task from the thread, checks it for NULL, and then reads the task again from the 
thread. in the meantime, the compiler thread can set the task back to 
NULL so that the check was pointless.

The task must be read only once.  The field _task must be volatile, so that
the C-compiler can't re-introduce the read.  

Please review this change. I please need a sponsor. I think this should be fixed 
in 10.
http://cr.openjdk.java.net/~goetz/wr18/8194482-fixPrintCompThr/webrev.01/

Best regards,
  Goetz.