[riscv-port-jdk17u:riscv-port] RFR: 8295270: RISC-V: Clean up and refactoring for assembler functions

[Gui Cao]

Please review this backport to riscv-port-jdk17u.
Backport of [JDK-8295270](https://bugs.openjdk.org/browse/JDK-8295270).
[JDK-8295270](https://bugs.openjdk.org/browse/JDK-8295270) not clean，origin patch has no eden_allocate, which removed with [JDK-8290706](https://bugs.openjdk.org/browse/JDK-8290706)

Testing:

Tier1-3 passed without new failure on unmacthed (release).

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Commit messages:
 - backport 8295270: RISC-V: Clean up and refactoring for assembler functions

Changes: https://git.openjdk.org/riscv-port-jdk17u/pull/38/files
 Webrev: https://webrevs.openjdk.org/?repo=riscv-port-jdk17u&pr=38&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8295270
  Stats: 1361 lines in 12 files changed: 641 ins; 609 del; 111 mod
  Patch: https://git.openjdk.org/riscv-port-jdk17u/pull/38.diff
  Fetch: git fetch https://git.openjdk.org/riscv-port-jdk17u.git pull/38/head:pull/38

PR: https://git.openjdk.org/riscv-port-jdk17u/pull/38