[POWERPC] Adjust TASK_SIZE on ppc32 systems to 3GB that are capable

All ppc32 systems except PReP and 8xx are capable of handling 3G of user
address space.  Old legacy had set this to 2GB and no one has bothered to
fix it.

8xx could be bumped up to 3GB if its SW TLB miss handlers were fixed up
to properly determine kernel/user addresses.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>