[riscv-port-jdk11u:riscv-port] RFR: 8293566: RISC-V: Clean up push and pop registers

[Gui Cao]

Hi, I would like to backport [JDK-8293566](https://bugs.openjdk.org/browse/JDK-8293566) to riscv-port-jdk11u. with this patch ,it will have better readability. jdk11u not have z gc, so this backport is not clean.

### Testing
- [ ] Run tier1 tests on SOPHON SG2042 (release)

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Commit messages:
 - Backport 8293566: RISC-V: Clean up push and pop registers

Changes: https://git.openjdk.org/riscv-port-jdk11u/pull/36/files
  Webrev: https://webrevs.openjdk.org/?repo=riscv-port-jdk11u&pr=36&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8293566
  Stats: 35 lines in 4 files changed: 7 ins; 3 del; 25 mod
  Patch: https://git.openjdk.org/riscv-port-jdk11u/pull/36.diff
  Fetch: git fetch https://git.openjdk.org/riscv-port-jdk11u.git pull/36/head:pull/36

PR: https://git.openjdk.org/riscv-port-jdk11u/pull/36