Shenandoah: SIGBUS in load_reference_barrier_native

This readily happens on AArch64 and jcstress with Shenandoah enabled after JDK-8232010.

$ wget https://builds.shipilev.net/jcstress/jcstress-tests-all-20191016.jar
$ build/linux-aarch64-server-fastdebug/images/jdk/bin/java -jar jcstress-tests-all-20191016.jar --jvmArgsPrepend "-XX:+UnlockExperimentalVMOptions -XX:+UnlockDiagnosticVMOptions -XX:+UseShenandoahGC" -m quick -t CAE

# A fatal error has been detected by the Java Runtime Environment:
#
# SIGBUS (0x7) at pc=0x0000ffffbd2cc2e4, pid=29007, tid=29010
#
# JRE version: OpenJDK Runtime Environment (14.0) (fastdebug build 14-internal+0-adhoc.shade.jdk-jdk)
# Java VM: OpenJDK 64-Bit Server VM (fastdebug 14-internal+0-adhoc.shade.jdk-jdk, mixed mode, sharing, compressed oops, shenandoah gc, linux-aarch64)
# Problematic frame:
# V [libjvm.so+0x10bc2e4] ShenandoahHeap::cas_oop(oop, oop*, oop)+0x70
#
# No core dump will be written. Core dumps have been disabled. To enable core dumping, try "ulimit -c unlimited" before starting Java again
#
# If you would like to submit a bug report, please visit:
# http://bugreport.java.com/bugreport/crash.jsp
#

--------------- S U M M A R Y ------------

Command Line: -XX:+UnlockExperimentalVMOptions -XX:+UnlockDiagnosticVMOptions -XX:+UseShenandoahGC -XX:ShenandoahGCHeuristics=aggressive -XX:-TieredCompilation -XX:+UnlockDiagnosticVMOptions -XX:+StressLCM -XX:+StressGCM org.openjdk.jcstress.vm.SimpleTestMain

Host: merino.lab2.eng.bos.redhat.com, AArch64, 46 cores, 95G, Red Hat Enterprise Linux Server release 7.6 (Maipo)
Time: Mon Oct 21 07:28:00 2019 EDT elapsed time: 0 seconds (0d 0h 0m 0s)

--------------- T H R E A D ---------------

Current thread (0x0000ffffb4061000): JavaThread "main" [_thread_in_Java, id=29010, stack(0x0000ffffbbf50000,0x0000ffffbc150000)]

Stack: [0x0000ffffbbf50000,0x0000ffffbc150000], sp=0x0000ffffbc14d2f0, free space=2036k
Native frames: (J=compiled Java code, A=aot compiled Java code, j=interpreted, Vv=VM code, C=native code)
V [libjvm.so+0x10bc2e4] ShenandoahHeap::cas_oop(oop, oop*, oop)+0x70
V [libjvm.so+0x12a3594] oop ShenandoahBarrierSet::load_reference_barrier_native_impl<oop>(oop, oop*)+0x1ac
V [libjvm.so+0x129f674] ShenandoahBarrierSet::load_reference_barrier_native(oop, oop*)+0x44
V [libjvm.so+0x1344ca8] ShenandoahRuntime::load_reference_barrier_native(oopDesc*, oop*)+0x108