[sh/11] 8239926: Shenandoah: Shenandoah needs to mark nmethod's metadata

[Zhengyu Gu]

I would like to backport this patch to Shenandoah JDK11u.

The same problems exist in sh/jdk11u, although, it is much harder to 
reproduce, given it performs a lot less class unloading with default 
settings.

I managed to reproduce the problems with following parameters:
-XX:+UnlockExperimentalVMOptions -XX:+UseShenandoahGC 
-XX:ShenandoahUnloadClassesFrequency=1 
-XX:+ClassUnloadingWithConcurrentMark -XX:+ClassUnloading



Original Bug: https://bugs.openjdk.java.net/browse/JDK-8239926
sh/11u Webrev: 
http://cr.openjdk.java.net/~zgu/JDK-8239926/sh-jdk11/webrev.00/

Test:
   hotspot_gc_shenandoah
   tools/javac with above parameters (normal and traversal)

Thanks,

-Zhengyu