[jdk23u] RFR: 8342014: RISC-V: ZStoreBarrierStubC2 clobbers rflags

[Gui Cao]

Hi, The same issue also exists in the jdk23u. I would like to backport this to jdk23u. becasue jdk23u not have [JDK-8334060](https://bugs.openjdk.org/browse/JDK-8334060), so this backport is not clean. This is a risc-v specific change, risk is low.

# Testing
- [ ]  Run tier1-3 tests on SOPHON SG2042 (release)

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Commit messages:
 - Backport 8342014: RISC-V: ZStoreBarrierStubC2 clobbers rflags

Changes: https://git.openjdk.org/jdk23u/pull/176/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk23u&pr=176&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8342014
  Stats: 30 lines in 2 files changed: 0 ins; 0 del; 30 mod
  Patch: https://git.openjdk.org/jdk23u/pull/176.diff
  Fetch: git fetch https://git.openjdk.org/jdk23u.git pull/176/head:pull/176

PR: https://git.openjdk.org/jdk23u/pull/176