m68knommu: allow elf_fdpic loader to be selected

The m68k architecture code is capable of supporting the binfmt_elf_fdpic
loader, so allow it to be configured. It is restricted to nommu
configurations at this time due to the MMU context structures/code not
supporting everything elf_fdpic needs when MMU is enabled.

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