ntdll: give user an error message if WINEARCH is not win32/win64

[Austin English]

Fixes https://bugs.winehq.org/show_bug.cgi?id=41378

See also https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=838474

-- 
-Austin
GPG: 14FB D7EA A041 937B
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