[riscv-port-jdk17u:riscv-port] RFR: 8307651: RISC-V: stringL_indexof_char instruction has wrong format string

[Feilong Jiang]

Hi,
Please review this clean backport of [JDK-8307651](https://bugs.openjdk.org/browse/JDK-8307651) to riscv-port-jdk17u.

backport is trivial since it only modifies some format of C2 string instructs.

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Commit messages:
 - 8307651: RISC-V: stringL_indexof_char instruction has wrong format string

Changes: https://git.openjdk.org/riscv-port-jdk17u/pull/56/files
 Webrev: https://webrevs.openjdk.org/?repo=riscv-port-jdk17u&pr=56&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8307651
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/riscv-port-jdk17u/pull/56.diff
  Fetch: git fetch https://git.openjdk.org/riscv-port-jdk17u.git pull/56/head:pull/56

PR: https://git.openjdk.org/riscv-port-jdk17u/pull/56