RFR: 8261579: AArch64: Support for weaker memory ordering in Atomic

[Andrew Haley]

At present the Atomic operations in HotSpot only support conservative (the very strongest) and relaxed (the weakest) memory ordering.
We should add at least seq_cst for LSE. This patch also adds a release-only CAS, needed for Shenandoah.

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Commit messages:
 - Support atomic_memory_order for CAS on BSD
 - Release-only CAS
 - AArch64 Atomic::cmpxchg acq_rel and seq_cst

Changes: https://git.openjdk.java.net/jdk/pull/4597/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4597&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261579
  Stats: 93 lines in 6 files changed: 88 ins; 2 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4597.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4597/head:pull/4597

PR: https://git.openjdk.java.net/jdk/pull/4597