This refactoring should make it easier to add new calling conventions.

This refactoring should make it easier to add new calling conventions.

Replace ffi_call_STDCALL and ffi_call_SYSV by a ffi_call_x86 function
that cleans up the stack when FFI_SYSV is used, and does nothing for
FFI_STDCALL.

Remove libffi_msvc\win32.S, which is out of date and also unused; it
was only used for building ctypes with the MingW compiler.