RFR: Uses keep_alive_barrier to keep deduped char array alive

[Zhengyu Gu]

We need to use newly introduced keep_alive_barrier to ensure deduped 
string's char array alive, as required by new concurrent partial GC.

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


Test:

   hotspot_gc_shenandoah (x64 and aarch64)


Thanks,

-Zhengyu