m68k: merge mmu and non-mmu versions of muldi3

The implementation of gcc's muldi3 support function differs only in
the use of the machine's 64 bit sized mul or not. (It isn't based
on using an MMU or not). Merge the current mmu and non-mmu versions
of arc/m68k/lib/muldi3 and use the appropriate pre-processor
conditionals to get the right version for all m68k processor types.

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