va_start is not supported on sysv abi?

[Inori]

Compiled on Win64, using clang-cl.
Test code:

int __attribute__((sysv_abi)) my_sprintf_s(char *buffer, size_t sizeOfBuffer, const char *format, ...)
{
	va_list arg_list;
	va_start(arg_list, format);
	int ret = vsprintf_s(buffer, sizeOfBuffer, format, arg_list);
	va_end(arg_list);

	//__builtin_va_list v_list;
	//__builtin_va_start(v_list, format);
	//int ret = vsprintf_s(buffer, sizeOfBuffer, format, v_list);
	//__builtin_va_end(v_list);

	return ret;
}

Neither will compile, I get:
error : 'va_start' used in System V ABI function

How can I deal with this?