A bit of dependency disentanglement for the generator programs

[DJ Delorie]

> (Since it takes longer than any other single operation, it's desirable
> to make it run as early as possible so that parallel builds are
> unlikely to get stuck waiting for it.)  genattrtab and genextract are

FYI for libiberty we addressed this issue somewhat by adjusting the
dependencies so that the longest compile times are scheduled to be
built first.  Thus, worst case is that one cpu builds one file the
whole time that the other cpu is building other things.