[riscv-port-jdk17u:riscv-port] RFR: 8294012: RISC-V: get/put_native_u8 missing the case when address&7 is 6

[Gui Cao]

Please review this backport to riscv-port-jdk17u.
Backport of [JDK-8294012](https://bugs.openjdk.org/browse/JDK-8294012). Applies cleanly.

Testing:

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

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Commit messages:
 - backport RISC-V: get/put_native_u8 missing the case when address&7 is 6

Changes: https://git.openjdk.org/riscv-port-jdk17u/pull/46/files
 Webrev: https://webrevs.openjdk.org/?repo=riscv-port-jdk17u&pr=46&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8294012
  Stats: 2 lines in 1 file changed: 2 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/riscv-port-jdk17u/pull/46.diff
  Fetch: git fetch https://git.openjdk.org/riscv-port-jdk17u.git pull/46/head:pull/46

PR: https://git.openjdk.org/riscv-port-jdk17u/pull/46