RFR: Offer termination before leaving on cancelled GC

[Roman Kennke]

All exit paths from the work-loop in partial GC must offer termination
on cancelled GC, otherwise a thread might get stück waiting for
early-leavers.

http://cr.openjdk.java.net/~rkennke/checkhandlecancelledgc/webrev.00/
<http://cr.openjdk.java.net/%7Erkennke/checkhandlecancelledgc/webrev.00/>

Roman