RFR: 8338019: Fix simple -Wzero-as-null-pointer-constant warnings in riscv code

[Gui Cao]

Hi, 
Consistent with the aarch64 issue [JDK-8337786](https://bugs.openjdk.org/browse/JDK-8337786), a similar build warning exists in riscv. Please help review this trivial change that replaces some uses of literal 0 as a null pointer constant in riscv code to instead use nullptr.

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

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Commit messages:
 - 8338019: Fix simple -Wzero-as-null-pointer-constant warnings in riscv code

Changes: https://git.openjdk.org/jdk/pull/20506/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20506&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8338019
  Stats: 5 lines in 4 files changed: 0 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/20506.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20506/head:pull/20506

PR: https://git.openjdk.org/jdk/pull/20506