RFR: 8266440: Shenandoah: TestReferenceShortcutCycle.java test failed on AArch64

[Zhengyu Gu]

JDK-8263427 missed corresponding changes for aarch64.

LRB needs to be enabled during weak roots/references processing, regardless if heap is stable.

Test:

- [x] hotspot_gc_shenandoah on Macosx/AArch64
- [ ] hotspot_gc_shenandoah on Linux 64/AArch64

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Commit messages:
 - update
 - v1

Changes: https://git.openjdk.java.net/jdk/pull/3902/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3902&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8266440
  Stats: 15 lines in 1 file changed: 14 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3902.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3902/head:pull/3902

PR: https://git.openjdk.java.net/jdk/pull/3902