m68knommu: add definitions to support elf_fdpic program loader

Add a few required defines to support using the binfmt_elf_fdpic
loader on the m68k architecture. The values are defined to be consistent
with those used on arm and sh which support this too.

The most important m68k specific change is the register initialization.
The pt_reg structure only contains a subset of the architecture general
registers, so we are more limited than to be expected on what can be used.

Signed-off-by: Greg Ungerer <gerg@linux-m68k.org>