powerpc: Don't include KERNBASE in genassym, it's unnecessary

A related future change, which changes KERNBASE for Book-E for some reason
causes a "KERNBASE redefined" error with assym.inc, even though it only changed
the value of KERNBASE and nothing else. Since machine/vmparam.h is already
included in booke/locore.S, and the requisite guards are already in place for
properly handling KERNBASE in vmparam.h, just remove it from genassym, and
include vmparam.h in the AIM locore files.