[riscv-port] RFR: 8276988: riscv: Fix a register name string: lr to x5

[zhengxiaolinX]

Hi team -

Simply fix a register name string: `lr` is `names[1]` so `names[5]` should be `x5`.

Thanks,
Xiaolin

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Commit messages:
 - Fix the register name lr to x5

Changes: https://git.openjdk.java.net/riscv-port/pull/2/files
 Webrev: https://webrevs.openjdk.java.net/?repo=riscv-port&pr=2&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8276988
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/riscv-port/pull/2.diff
  Fetch: git fetch https://git.openjdk.java.net/riscv-port pull/2/head:pull/2

PR: https://git.openjdk.java.net/riscv-port/pull/2