[jdk19u] RFR: 8296448: RISC-V: Fix temp usages of heapbase register killed by MacroAssembler::en/decode_klass_not_null

[Xiaolin Zheng]

A backport from 20 for RISC-V backend: https://github.com/openjdk/jdk/commit/93fed9b251c21f20b68ddc4e179d6595275dbcd2

Only affects RISC-V backend, so impact is low.

The `access_store_at`'s signature changed so when cherry-picked from JDK20 I met one minor conflict (Not sure if it would be considered clean then).

Tier1~2 testing passed.

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Commit messages:
 - 8296448: RISC-V: Fix temp usages of heapbase register killed by MacroAssembler::en/decode_klass_not_null

Changes: https://git.openjdk.org/jdk19u/pull/71/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk19u&pr=71&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8296448
  Stats: 52 lines in 8 files changed: 7 ins; 6 del; 39 mod
  Patch: https://git.openjdk.org/jdk19u/pull/71.diff
  Fetch: git fetch https://git.openjdk.org/jdk19u pull/71/head:pull/71

PR: https://git.openjdk.org/jdk19u/pull/71