RFR: 8330266: RISC-V: Restore frm to RoundingMode::rne after JNI

[Hamlin Li]

Hi,
Can you help to review this patch?
It's exactly the same as https://github.com/openjdk/jdk/pull/18785 which is withdrawn, the reason could be that I deleted the branch `restore-frm-after-jni` after I `integrate` the pr, but in fact before it's indeed integrated by github.
Sorry for the inconvenience.

Thanks

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Commit messages:
 - minor refinement
 - refine code
 - fix typo
 - Initial commit

Changes: https://git.openjdk.org/jdk/pull/18839/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18839&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8330266
  Stats: 25 lines in 5 files changed: 25 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/18839.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18839/head:pull/18839

PR: https://git.openjdk.org/jdk/pull/18839