m68k: merge the MMU and non-MMU signal.c code

The MMU (signal_mm.c) and non-MMU (signal_no.c) versions of the m68k
architecture signal handling code are very similar. Most of their code is
the same.

Merge the two back into a single signal.c, and move some of the code around
inside the file to minimize the number of #ifdefs required. Specificially
we can group out the CONFIG_FPU and the CONFIG_MMU code. We end up needing
a few other "#ifdef CONFIG_MMU" as well, but not too many.

Signed-off-by: Greg Ungerer <gerg@uclinux.org>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>