RFR: 8368724: RISC-V: Check if unaligned_access is enabled before use

[Anjian Wen]

we can notice that the `unaligned_access.value()` can only be used when the `unaligned_access.enabled()` is true, so we add the check here before use

Manually checked the result on platforms w/wo fast misaligned accesses by running: $java -XX:+PrintFlagsFinal -version | grep AvoidUnalignedAccess

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Commit messages:
 - RISC-V: Add unaligned access check

Changes: https://git.openjdk.org/jdk/pull/27510/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27510&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8368724
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/27510.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27510/head:pull/27510

PR: https://git.openjdk.org/jdk/pull/27510