[14] RFR 8239926: Shenandoah: Shenandoah needs to mark nmethod's metadata

[Zhengyu Gu]

I would like to backport this patch to 14u, as it is a fatal bug.

Original bug: https://bugs.openjdk.java.net/browse/JDK-8239926
Original webrev: http://cr.openjdk.java.net/~zgu/JDK-8239926/webrev.04/


14u webrev: http://cr.openjdk.java.net/~zgu/JDK-8239926/14u/webrev.00/

Test:
   hotspot_gc_shenandoah
   tier1 with Shenandoah GC

Thanks,

-Zhengyu