RFR: 8311074: RISC-V: Fix -Wconversion warnings in some code header files

[Gui Cao]

Hi, refer to the modification of aarch64 in https://github.com/openjdk/jdk/pull/14675 , here is the riscv version, before this PR has the same -Wconversion warnings as aarch64, after this this PR the -Wconversion warnings has been eliminated.
Please take a look and have some reviews. Thanks a lot.

## Testing:
- [x] Tier1 tests (release)

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Commit messages:
 - Merge branch 'master' into JDK-8311074
 - 8311074: RISC-V: Fix -Wconversion warnings in some code header files

Changes: https://git.openjdk.org/jdk/pull/14702/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14702&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8311074
  Stats: 4 lines in 3 files changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/14702.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14702/head:pull/14702

PR: https://git.openjdk.org/jdk/pull/14702