The OS ABI basically defines what the compiler will do. While it's possible to run a compiler in ILP64 mode on Windows 64, you won't get too far if you try to pass a 64-bit integer >4^32 into a Windows system call.
... or pass any 64-bit integer. Windows API calls are mostly "stdcall" and use stack for parameter passing - pushing 8 bytes instead of 4 could be disasterous in many ways - especially considering that the callee cleans the stack in this case.
There is no stdcall/ccall distinction on 64-bit Windows, there is only one ABI convention that sadly uses a different set of registers for parameter passing than Linux. Only the first four parameters uses registers, the rest is passed on the stack.
