gh-145410: Fix sysconfig.get_platform() on Windows when sys.version is truncated

[04cb]

Fixes #145410

The function was checking for 'amd64' in sys.version to detect 64-bit
Windows, but sys.version is truncated at a fixed width and may not
contain the architecture marker when the compiler version string is long.

Use platform.machine() instead, which reliably returns the machine
architecture on Windows regardless of the sys.version content.

• Issue: sysconfig.get_platform() could return wrong value because sys.version is truncated #145410

[bedevere-app]

Most changes to Python require a NEWS entry. Add one using the blurb_it web app or the blurb command-line tool.

If this change has little impact on Python users, wait for a maintainer to apply the skip news label instead.

[python-cla-bot]

All commit authors signed the Contributor License Agreement.

[chris-eibl]

Quoting @zooba from the issue #145410 (comment)

sysconfig should rely on _sysconfig if it needs access to something known at compile time.

I also don't think this is the way to go. Furthermore, CI hints that there are problems to solve ...

[zooba]

This is not the way - platform and sysconfig have different purposes, so they shouldn't mix.

Add a compile-time constant to _sysconfig for this.

[bedevere-app]

A Python core developer has requested some changes be made to your pull request before we can consider merging it. If you could please address their requests along with any other requests in other reviews from core developers that would be appreciated.

Once you have made the requested changes, please leave a comment on this pull request containing the phrase I have made the requested changes; please review again. I will then notify any core developers who have left a review that you're ready for them to take another look at this pull request.