[riscv-port-jdk11u:riscv-port] RFR: 8285303: riscv: Incorrect register mask in call_native_base

[Gui Cao]

Hi, please help review this backport to riscv-port-jdk11u.
Backport of [JDK-8285303](https://bugs.openjdk.org/browse/JDK-8285303). Applies cleanly.

Testing:

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

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Commit messages:
 - Backport 8285303: riscv: Incorrect register mask in call_native_base

Changes: https://git.openjdk.org/riscv-port-jdk11u/pull/18/files
  Webrev: https://webrevs.openjdk.org/?repo=riscv-port-jdk11u&pr=18&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8285303
  Stats: 19 lines in 2 files changed: 0 ins; 17 del; 2 mod
  Patch: https://git.openjdk.org/riscv-port-jdk11u/pull/18.diff
  Fetch: git fetch https://git.openjdk.org/riscv-port-jdk11u.git pull/18/head:pull/18

PR: https://git.openjdk.org/riscv-port-jdk11u/pull/18