m68knommu: remove size limit on non-MMU TASK_SIZE

The TASK_SIZE define is used in some places as a limit on the size of
the virtual address space of a process. On non-MMU systems those addresses
used in comparison will be physical addresses, and they could be anywhere
in the 32bit physical address space. So for !CONFIG_MMU systems set the
TASK_SIZE to the maximum physical address.

Signed-off-by: Greg Ungerer <gerg@uclinux.org>