python: fix mingw python detection code in case pkg-config isn't available

The code assumed that sysconfig.get_platform() returns "mingw" for mingw Python,
but that's no longer the case for 2.5 years now, as it now only starts with
"mingw" and contains further information like the arch and other ABI relevant things
to avoid conflicts.

This updates the detection code to the current status quo. mingw Python only documents
right now that it starts with "mingw", and none of that arch stuff, but it's
unlikely that this will change, and this looks less error prone than looking at CC.

Fixes #12547

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