RFR: Fix freeze when running OOM during write barrier

[Roman Kennke]

We sometimes freeze when a write-barrier runs out of memory. Reason is
the recent refactoring in our driver thread: we would skip turning off
evacuation, however Java threads are waiting for this to happen.
They'll wait indefinitely, and thus never return to a safepoint.

http://cr.openjdk.java.net/~rkennke/fixfreeze/webrev.00/

Ok to push?

Roman