[8u] RFR 8217785: Padding ParallelTaskTerminator::_offered_termination variable

[Zhengyu Gu]

I would like backport the patch to JDK8u. This minor fix improves 
termination latency for multiple GC threads.

The patch does not apply cleanly to 8u, mainly due to missing macro 
DEFINE_PAD_MINUS_SIZE, (which introduced by DK-8049737) and different 
data type for _offered_termination variable.

Original bug: https://bugs.openjdk.java.net/browse/JDK-8217785
Original webrev: http://cr.openjdk.java.net/~zgu/JDK-8217785/webrev.00/

8u backport:
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8217785-8u/webrev.00/

Thanks,

-Zhengyu