RFR (XS): Relax assert to not fire at safepoint

[Roman Kennke]

We're checking that a non-GC thread going into evacuate_object() does
not hold the Threads_lock to prevent deadlock later.

However, it can happen that the VMThread gets there via
biased-locking-revokation, and this guy does hold the Threads_lock when
it does. It seems ok to me to get there in this case. It will end up
waiting for evacuation to stop, which should also be ok, since we
*don't* stop GC threads on non-GC pauses, and those will eventually turn
off evacuation.

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

Ok?

Roman