[riscv-port-jdk11u:riscv-port] RFR: 8305006: Use correct register in riscv_enc_fast_unlock()

[Gui Cao]

Hi, The same issue also exists in the riscv-port-jdk11u. It's already fixed in the jdk17u version above, for performance reasons, I'd like to backport it to this repo.

Testing:

- [ ] Run tier1 tests on SOPHGO SG2042 (release)

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Commit messages:
 - Backport 8305006: Use correct register in riscv_enc_fast_unlock()

Changes: https://git.openjdk.org/riscv-port-jdk11u/pull/8/files
 Webrev: https://webrevs.openjdk.org/?repo=riscv-port-jdk11u&pr=8&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8305006
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/riscv-port-jdk11u/pull/8.diff
  Fetch: git fetch https://git.openjdk.org/riscv-port-jdk11u.git pull/8/head:pull/8

PR: https://git.openjdk.org/riscv-port-jdk11u/pull/8