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Are functions with more than 6 arguments really forbidden on x86-64? How do things like CreateFile or CreateWindow from the Windows API get compiled then?

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That's the syscall calling convention, not the C one (though it's more like a platform ABI than something exclusive to C these days). In that, on Linux x64 all the args after the first six are pushed to the stack, on Windows it's after four args.

It's probably worth clarifying that on 64-bit Windows, the first first four arguments are passed in registers regardless of type, and the remainder (if any) are pushed to the stack. But on 64-bit Linux/BSD/OSX, depending on the mix of types, you might have as many as 14 register arguments. You get 6 "integer" registers, plus 8 "vector" registers, where "vector" registers are also used for floating point.

http://www.agner.org/optimize/calling_conventions.pdf

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