RFR: 8343030: RISC-V: Small assembler cleanups

[Fei Yang]

Hi, please review this small code cleanups.

Witnessed some explicit type conversion to `address` is not necessary now.
Also we can make use of `void la(Register Rd, const address addr)` where possible to simplify the code.
This also adds one temporary register parameter for the `cmpptr` assembler routine.

Testing on linux-riscv64:
- [ ] tier1 (release)

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Commit messages:
 - 8343030: RISC-V: Small assembler cleanups

Changes: https://git.openjdk.org/jdk/pull/21699/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21699&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8343030
  Stats: 33 lines in 8 files changed: 2 ins; 11 del; 20 mod
  Patch: https://git.openjdk.org/jdk/pull/21699.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21699/head:pull/21699

PR: https://git.openjdk.org/jdk/pull/21699