fix: avoid unsafe UTF-16 fast paths on aarch64 JDK 8

[xuzhaorui]

What this PR does / why we need it?

On AArch64 with Oracle JDK 8, C2 can miscompile unaligned Unsafe accesses used by the UTF-16 parsing and writing fast paths. This can make repeated parsing of false produce an incorrect value and can corrupt repeated UTF-16 string serialization.

This change disables only the affected fast paths on that runtime and keeps the existing optimized paths for other environments.

Summary of your change

• Add AARCH64_JDK8 runtime detection for JDK 8 AArch64, excluding Android, GraalVM, and OpenJ9.
• Use character-by-character comparison for IOUtils.isALSE(char[]) and notALSE(char[]) on the affected runtime.
• Route JDK 8 AArch64 UTF-16 writers to the safe implementation instead of the Unsafe-based implementation.
• Add regression coverage for repeated boolean parsing and string serialization.

Related: #7834. This PR focuses on the boolean parsing and UTF-16 string serialization paths reported in #7838; it does not change the other Unsafe access sites covered by #7834.

本地环境为 x86_64 + JDK 17，无法直接复现 AArch64 + JDK 8 的 C2 错误；因此回归测试在当前环境验证行为不回退，目标平台上的安全分支由运行时检测启用。

Validation:

• ./mvnw -pl core test — 7990 tests passed.
• ./mvnw -pl core -Dfastjson2.creator=reflect -Dtest=Issue7838Test test — 2 tests passed.
• ./mvnw -pl core -Dtest=Issue7838Test test — 2 tests passed.
• ./mvnw -pl core validate — passed with 0 Checkstyle violations.

No documentation changes are needed.

Please indicate you've done the following:

• Made sure tests are passing and test coverage is added if needed.
• Made sure commit message follow the rule of Conventional Commits specification.
• Considered the docs impact and opened a new docs issue or PR with docs changes if needed.

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