this change is in preparation for amd64. the systab calling
convention was also changed to return uintptr (as segattach
returns a pointer) and the arguments are now passed as
va_list which handles amd64 arguments properly (all arguments
are passed in 64bit quantities on the stack, tho the upper
part will not be initialized when the element is smaller
than 8 bytes).
this is partial. xalloc needs to be converted in the future.
