set VSCMD_ARG_TGT_ARCH based on targetted architecture

[MusicalNinjaDad]

This fixes #1861 and any other build setups which make use of setuptools._distutils.util.get_platform() on windows

Approach

• Uses dict lookup pattern analog to many other places within the code
• Only sets the variable if it has not already been set manually. Raises a FatalError if the variable is preset to a different value than expected.
• Will overwrite a pre-set VSCMD_ARG_TGT_ARCH=""
• Tests monkeypatch the environment before calling setuptools._distutils.util.get_platform() and validating returned platform

Test availability

• Tests are set to only run on windows
• monkeypatching os.name in a tight context worked locally (Win11 + WSL2 + Ubunutu 22.04), but fails in pipeline on github actions
• all tests pass on github actions windows runner but tests targeting arm64 fail on azure pipeline. I'm assuming this is a weird, azure-specific thing in their environment and have set the arm64 test to skip on azure

Potential refactoring idea for a future PR

• I'd like to refactor the various mapping lookups into a WindowsArchitecture dataclass - there are many places with the pattern somevar = architecture_mapping_dict[arch] which would be more readable as somevar = targetarchitecure.somevar

[MusicalNinjaDad]

@henryiii - I'd welcome your feedback on this suggestion please.

[henryiii]

I've always been in favor of this, it's just been others that are worried that setting this variable will make some build backend confused about not having the others.

[henryiii]

@joerick or @mayeut, thoughts?

[mayeut]

I don't know enough to really have an informed opinion here.

[joerick]

I'm okay with this, it seems to me to be very similar to _PYTHON_HOST_PLATFORM/ARCHFLAGS that we set in macos.py.

I don't have any specific knowledge of the MS compiler to predict and second-order effects though. @zooba, since you created the cross-compilation mechanism, do you have an opinion on the use of VSCMD_ARG_TGT_ARCH for cross-compilation?

[zooba]

This seems harmless, provided we aren't replacing an existing value for the variable. If it's been set, we have to assume that cross-compilation is configured across a range of settings (particularly LIB and PATH environment variables) and bail out (or use it to automatically select the set of builds to run...?)

Other than that, seems fine. I don't think anything but setuptools takes this into account, but it's as "standard" as things get. Anything else will be backend-specific.

[MusicalNinjaDad]

I think this is done now (?) - I've updated the tests to expect the FatalError and can't think of anything else to do in the main code

[joerick]

Looks good to me!