m68k: always make available dump_fpu()

Our local m68k architecture dump_fpu() is conditionally compiled in on
CONFIG_FPU. That is OK for all existing MMU enabled CPU types, but won't
handle the case for some ColdFire SoC CPU parts that we want to support
that have no FPU hardware.

dump_fpu() is expected to be present by the ELF loader, so we must always
have it available and exported.

Remove the conditional and reorganize the dump_fpu hard FPU code path
to let the compiler remove code when not needed.

This change based on changes and discussion from Yannick Gicquel
<yannick.gicquel@open.eurogiciel.org>.

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