RFR(XS): Shenandoah support in jvmci

[Zhengyu Gu]

Please review the simple changes that fix most of jvmci test failures.

Webrev:http://cr.openjdk.java.net/~zgu/jvmci/webrev/ 
<http://cr.openjdk.java.net/%7Ezgu/jvmci/webrev/>


Thanks,

-Zhengyu