[8] RFR: Shenandoah: JNI weak roots are always cleared before Full GC mark

[Aleksey Shipilev]

Amir found the crash with Cassandra that started to happen recently with Shenandoah 8u. It
reproduces more or less reliably with starting Cassandra 3.11.4 with fastdebug build and
-XX:ShenandoahGCMode=passive -XX:-ShenandoahDegeneratedGC.

This is a symptom of a larger problem (see JDK-8248041), but 8u it is catastrophic due to clearing
of the weak roots.

Fix:
  https://cr.openjdk.java.net/~shade/shenandoah/8u-fullgc-jni-weak/webrev.01/

Testing: hotspot_gc_shenandoah; Cassandra tests

-- 
Thanks,
-Aleksey