RFR: 8353009: Document target selection flag for Windows AArch64 builds

[Saint Wesonga]

The target selection configuration flag for Windows AArch64 should be added to the build documentation for improved discoverability and completeness.

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Commit messages:
 - Document how to build for Windows AArch64

Changes: https://git.openjdk.org/jdk/pull/24267/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24267&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8353009
  Stats: 18 lines in 2 files changed: 18 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/24267.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24267/head:pull/24267

PR: https://git.openjdk.org/jdk/pull/24267