m68k: remove arch specific non-optimized memcmp()

The m68k arch implements its own memcmp() function. It is not optimized
in any way (it is the most strait forward coding of memcmp you can get).
Remove it and use the kernels standard memcmp() implementation.

This also goes part of the way to fixing a regression caused by commit
ea61bc46 ("m68k/m68knommu: merge MMU and
non-MMU string.h"), which breaks non-coldfire non-mmu builds (which is
the 68x328 and 68360 families). They currently have no memcmp() function
defined, since there is none in the m68knommu/lib functions.

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