Remove the ia64 hackery in threadinit() that was needed to work around

Remove the ia64 hackery in threadinit() that was needed to work around
the lameness of the kstack code. The EPC overhaul de-lame-ified the
kstack code by removing the need for contigmalloc(). We can now
allocate stacks using malloc(). We probably want to make the stacks
swappable as well so that we can make it MI. But that's another story.