[riscv-port] RFR: 8277430: riscv: Refactor interpreter native wrappers

[Yadong Wang]

A similar refactor similar to https://bugs.openjdk.java.net/browse/JDK-8261071, to make the code more clean and open for extension.

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Commit messages:
 - 8277430: riscv: Refactor interpreter native wrappers

Changes: https://git.openjdk.java.net/riscv-port/pull/12/files
 Webrev: https://webrevs.openjdk.java.net/?repo=riscv-port&pr=12&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8277430
  Stats: 155 lines in 2 files changed: 39 ins; 33 del; 83 mod
  Patch: https://git.openjdk.java.net/riscv-port/pull/12.diff
  Fetch: git fetch https://git.openjdk.java.net/riscv-port pull/12/head:pull/12

PR: https://git.openjdk.java.net/riscv-port/pull/12