m68k: Fix build with -mcpu=68040 or higher (BZ 30740)

GCC currently does not define __mc68020__ for -mcpu=68040 or higher,
which memcpy/memmove assumptions.  Since this memory copy optimization
seems only intended for m68020, disable for other m680X0 variants.

Checked on a build for m68k-linux-gnu target mc68020 and mc68040.