RFR: Fix weakref precleaning

[Roman Kennke]

Weakref precleaning does infact need to do something in the
keepalive-closure:

- It must update internal refs in j.l.r.Reference instances. I.e. make
referent point to to-space
- It must mark through the next field for references that are enqueued

And therefore the complete_gc closure also needs to process stuff on the
marking stack.

The patch fixes those issues plus also drains SATB buffers, and adds a
ResourceMark for -Xlog:gc+ref logging.

http://cr.openjdk.java.net/~rkennke/fixpreclean/webrev.00/
<http://cr.openjdk.java.net/%7Erkennke/fixpreclean/webrev.00/>

Testing: hotspot_gc_shenandoah and some specjvm