RFR: Better handling of cancelled_gc

[Roman Kennke]

This patch:
- Makes _cancelled_gc volatile and accessed by load_acquire and
release_store so that observing threads are seeing changes quickly
instead of checking cached values
- Improve the concurrent mark loop so that it doesn't check that flag
on every iteration, but only every 1000th iteration, which should be
plenty fast enough

http://cr.openjdk.java.net/~rkennke/cancelgc/webrev.01/

OK?

Roman