RFR: 8261504: Shenandoah: reconsider ShenandoahJavaThreadsIterator::claim memory ordering

[Aleksey Shipilev]

JDK-8256298 added the thread iterator for thread roots, and I don't think we need the Hotspot's default memory_order_conservative, which emits two-way memory fences around the CASes at least on AArch64 and PPC64. The simple "relaxed" should do.

Additional testing:
 - [x] Linux x86_64 hotspot_gc_shenandoah
 - [x] Linux AArch64 hotspot_gc_shenandoah
 - [x] Linux AArch64 tier1 with Shenandoah

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Commit messages:
 - 8261504: Shenandoah: reconsider ShenandoahJavaThreadsIterator::claim memory ordering

Changes: https://git.openjdk.java.net/jdk/pull/2506/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2506&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261504
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2506.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2506/head:pull/2506

PR: https://git.openjdk.java.net/jdk/pull/2506