Add Windows ARM64 wheel support

[ssss141414]

Summary

• detect native win-arm64 Python builds without relying on pointer width
• build the CRT dependencies with Ninja and clang-cl
• disable only the vendored optimized checksum path that currently fails under ClangCL, retaining the portable implementation
• add native Windows ARM64 build/install tests across the stable-ABI and free-threaded Python matrix
• add a Windows ARM64 release wheel entry point matching the existing Win32/Win64 scripts

Validation

• built awscrt-0.36.2-cp313-abi3-win_arm64.whl on native Windows ARM64
• wheel SHA-256: AF14C5D014A29123E07F53CA9F615F96DEA3E21695791AD5EBBF1C1BCB2E5205
• _awscrt.pyd PE machine: 0xAA64
• clean installed-wheel local suite: 473 passed, 252 skipped
• additional remote HTTP/2 test currently receives HTTP 503 from the public endpoint instead of the expected 404; this is not treated as a pass

Release integration note

The new release script produces the ARM64 wheel set, but the external CodeBuild/CodePipeline source and aggregation wiring are intentionally not changed here. That infrastructure must provide and validate the ARM64 secondary source before the release tee consumes it.

[ssss141414]

Hosted Windows ARM64 validation is complete on final head ef45c356a7e8970eed3aa85306bf738150075b07.

Run: https://github.com/ssss141414/aws-crt-python/actions/runs/32470230302

All six native windows-11-arm build/install rows passed:

• CPython 3.11 wheel on 3.11 and 3.12 (cp311-abi3-win_arm64)
• CPython 3.13 wheel on 3.13 and 3.14 (cp313-abi3-win_arm64)
• free-threaded 3.13 (cp313-cp313t-win_arm64)
• free-threaded 3.14 (cp314-cp314t-win_arm64)

Each clean installed-wheel job ran 497 tests: OK (skipped=252). The workflow asserts the filename and embedded WHEEL tag, verifies that exactly the two known httpbin.org-dependent tests are excluded, and rejects empty discovery. Those two tests are isolated because the public endpoint currently returns 503 instead of their expected 404.

The run-wide failure is from pre-existing fork limitations: AWS OIDC credentials/CRT_CI_ROLE are unavailable to fork jobs, and some macOS jobs are blocked by account billing. The ARM64 jobs do not use those credentials and all passed. External CodeBuild/CodePipeline source and aggregation wiring remains a release-infrastructure follow-up as documented in the PR description.

[ssss141414]

Hi awscrt maintainers,
I'm from Microsoft and recently I'm working on improving Python ecosystem support for Windows on Arm. So I adds CI support for Windows ARM64. Could you please help to review? Thanks.