Make win32.mak compatible with MODEL=32omf

[MoonlightSentinel]

The previous assumption was MODEL=32 because MODEL=32mscoff is built by
win64.mak.

Verified locally because Azure requires further changes outside of this repository - see dlang/druntime#3703.

[dlang-bot]

Thanks for your pull request and interest in making D better, @MoonlightSentinel! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
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• My PR is fully covered with tests (you can see the coverage diff by visiting the details link of the codecov check)
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If your PR contains non-trivial changes, please reference a Bugzilla issue or create a manual changelog.

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + phobos#8367"

[RazvanN7]

Why is this a draft? Maybe I can force merge this one, but it needs to be a non-draft.

[RazvanN7]

Yep, I can force merge this one. Ping me when it's ready

[MoonlightSentinel]

Initially had hoped that the Druntime PR could be merged first - in case there are subtle difference between Azure and my local setup. But this should be good to go once all other jobs pass.