RFR: 8266522: Shenandoah: Shenandoah LRB calls wrong runtime barrier on aarch64

[Zhengyu Gu]

Shsenandoah LRB calls ShenandoahRuntime::load_reference_barrier_weak() on phantom access, which is wrong.

Should call ShenandoahRuntime::load_reference_barrier_phantom() instead.

Test:
  - [x] hotspot_gc_shenandoah on Linux aarch64

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Commit messages:
 - v1

Changes: https://git.openjdk.java.net/jdk/pull/3864/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3864&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8266522
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3864.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3864/head:pull/3864

PR: https://git.openjdk.java.net/jdk/pull/3864