Now the only thing you need to do to create a new syscall is write the
kernel function and put "//SYSCALL" immediately before it.

There are only two places we depended on these #defines: forkt and
initcode.  We now declare symbolic constants for the syscall numbers
in sysstubs, which makes forkt just work (though it gets filled in at
link time instead of by the preprocessor).  For initcode, we now link
initcode against sysstubs so it can get these constants.