m68k: remove duplicate memcpy() implementation

Merging the mmu and non-mmu directories we ended up with duplicate
implementations of memcpy(). One is a little more optimized for the
>= 68020 case, but that can easily be inserted into a single
implementation of memcpy(). Clean up the exporting of this symbol
too, otherwise we end up exporting it twice on a no-mmu build.

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