RFR: 8275226: Shenandoah: Relax memory constraint for worker claiming tasks/ranges

[Zhengyu Gu]

I believe for the cases that there is no plain read of atomic variables, we can relax memory ordering constraints for claiming tasks/ranges by workers.

Test:
- [x] hotspot_gc_shenandoah (fastdebug + release) on x86_64 and aarch64 Linux

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Commit messages:
 - v0

Changes: https://git.openjdk.java.net/jdk/pull/5929/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5929&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8275226
  Stats: 6 lines in 4 files changed: 0 ins; 0 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5929.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5929/head:pull/5929

PR: https://git.openjdk.java.net/jdk/pull/5929