[riscv-port-jdk17u:riscv-port] RFR: 8290496: riscv: Fix build warnings-as-errors with GCC 11

[Dingli Zhang]

Please review this backport to riscv-port-jdk17u.
Backport of [JDK-8290496](https://bugs.openjdk.java.net/browse/JDK-8290496). Applies cleanly.

Build without --disable-warnings-as-errors after this fix passed on unmatched board with GCC11.

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Commit messages:
 - backport JDK-8290496

Changes: https://git.openjdk.org/riscv-port-jdk17u/pull/2/files
 Webrev: https://webrevs.openjdk.org/?repo=riscv-port-jdk17u&pr=2&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8290496
  Stats: 43 lines in 2 files changed: 6 ins; 14 del; 23 mod
  Patch: https://git.openjdk.org/riscv-port-jdk17u/pull/2.diff
  Fetch: git fetch https://git.openjdk.org/riscv-port-jdk17u pull/2/head:pull/2

PR: https://git.openjdk.org/riscv-port-jdk17u/pull/2