[jdk19u] RFR: 8290496: riscv: Fix build warnings-as-errors with GCC 11

[Feilong Jiang]

Please review this backport to jdk19u.
Backport of [JDK-8290496](https://bugs.openjdk.java.net/browse/JDK-8290496). Applies cleanly. Approval is pending.

Build with `--disable-warnings-as-errors` after this fix passed on unmatched board with GCC11

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Commit messages:
 - Backport 5425573bb4de1a2434201bc7ec3700b527ce346b

Changes: https://git.openjdk.org/jdk19u/pull/40/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk19u&pr=40&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/jdk19u/pull/40.diff
  Fetch: git fetch https://git.openjdk.org/jdk19u pull/40/head:pull/40

PR: https://git.openjdk.org/jdk19u/pull/40