[riscv-port-jdk17u:riscv-port] RFR: 8302776: RISC-V: Fix typo CSR_INSTERT to CSR_INSTRET

[Gui Cao]

Please review this backport to riscv-port-jdk17u.
Backport of [JDK-8302776](https://bugs.openjdk.org/browse/JDK-8302776).

 Applies cleanly, and build success.

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Commit messages:
 - backport RISC-V: Fix typo CSR_INSTERT to CSR_INSTRET

Changes: https://git.openjdk.org/riscv-port-jdk17u/pull/42/files
 Webrev: https://webrevs.openjdk.org/?repo=riscv-port-jdk17u&pr=42&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8302776
  Stats: 2 lines in 2 files changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/riscv-port-jdk17u/pull/42.diff
  Fetch: git fetch https://git.openjdk.org/riscv-port-jdk17u.git pull/42/head:pull/42

PR: https://git.openjdk.org/riscv-port-jdk17u/pull/42