[Shenandoh 8u] Shenandoah string dedup thread is not properly initialized

[Zhengyu Gu]

This bug seems to exist since day one of 8u backport. The 
ConcurrentGCThread API is different in 8u and we leaves 
ShenandoahDedupThread not properly initialized before it enters work loop.

In Shenandoah String Deduplication tests, the bug results assertion 
failure that shows Thread::current() == NULL.

The bug only manifests on Windows, is due to discrepancy of java_start() 
implementation on different OSs. e.g. it sets *thread* on Linux.


Webrev: 
http://cr.openjdk.java.net/~zgu/shenandoah/8u_init_dedup_thread/webrev.00/

Test:
   hotspot_gc_shenandoah on Windows and Linux.

Thanks,

-Zhengyu