RFR: Missing jvmti weak roots in jdk8 and jdk9

[Zhengyu Gu]

Looks like we missed jvmti weak roots in jdk8 and 9.


JDK8: 
http://cr.openjdk.java.net/~zgu/shenandoah/sh-jdk8-jvmti-weak-root/webrev/

JDK9: 
http://cr.openjdk.java.net/~zgu/shenandoah/sh-jdk9-jvmti-weak-root/webrev.00/

Test:
   jdk8 and jdk9
   hotspot_gc-shenandoah Linux 64

Thanks,

-Zhengyu