RFR: Tacking termination of weakref processing

[Zhengyu Gu]

Weak reference processing also enters terminator, which was missed in 
earlier patch.

Also, fixed typo in early patch.

Webrev: http://cr.openjdk.java.net/~zgu/shenandoah/weakref_term/webrev.00/

Test:

   tier1_gc_shenandoah (fastdebug + release), with 
ShenandoahTerminationTrace on

Thanks,

-Zhengyu