[riscv-port-jdk11u:riscv-port] RFR: 8292867: RISC-V: Simplify weak CAS return value handling

[Dingli Zhang]

Please review this backport to riscv-port-jdk11u.
Backport of [JDK-8292867](https://bugs.openjdk.org/browse/JDK-8292867). Applies cleanly.

Testing:

- Tier1 passed without new failure on lp4a (release).

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Commit messages:
 - 8292867: RISC-V: Simplify weak CAS return value handling

Changes: https://git.openjdk.org/riscv-port-jdk11u/pull/31/files
  Webrev: https://webrevs.openjdk.org/?repo=riscv-port-jdk11u&pr=31&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8292867
  Stats: 39 lines in 2 files changed: 2 ins; 13 del; 24 mod
  Patch: https://git.openjdk.org/riscv-port-jdk11u/pull/31.diff
  Fetch: git fetch https://git.openjdk.org/riscv-port-jdk11u.git pull/31/head:pull/31

PR: https://git.openjdk.org/riscv-port-jdk11u/pull/31