[jdk21u] RFR: 8314117: RISC-V: Incorrect VMReg encoding in RISCV64Frame.java

[Feilong Jiang]

This fixes the incorrect VMReg encoding in `RISCV64Frame.java` for risc-v. Patch applies clean.

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Commit messages:
 - Backport 5c9162288570a140138a0055cd9c4e88fe40e69d

Changes: https://git.openjdk.org/jdk21u/pull/75/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk21u&pr=75&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8314117
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk21u/pull/75.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u.git pull/75/head:pull/75

PR: https://git.openjdk.org/jdk21u/pull/75