RFR: Handle single-threaded GC correctly

[Roman Kennke]

The problem that Aleksey reported comes from the fact that we
initialize the ReferenceProcessor as single-threaded, and this
repeatedly calls the same terminator. The fix makes ReferenceProcessor
work multithreaded, even when only using a single thread. This may not
be optimal, but it does the correct thing.

Ok to commit?
http://cr.openjdk.java.net/~rkennke/1threadedgc/wevrev.00/

Roman