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

[Erik Joelsson]

On Wed, 26 Mar 2025 22:38:40 GMT, Saint Wesonga <duke at openjdk.org> wrote:

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

I think this is fine for now, but ideally we should make configure understand that the build platform can be windows-aarch64 even if Cygwin is x86 running through emulation, so that it just works out of the box.

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Marked as reviewed by erikj (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/24267#pullrequestreview-2722993902