m68k: fix unused variable warning in mempcy.c

When compiling for original 68000 or ColdFire targets you will get the
following warning when compiling arch/m68k/lib/memcpy.c:

  CC      arch/m68k/lib/memcpy.o
arch/m68k/lib/memcpy.c: In function ‘__builtin_memcpy’:
arch/m68k/lib/memcpy.c:13:15: warning: unused variable ‘temp1’

This is easily fixed by moving the definition of temp1 into the code block
where it is used.

Signed-off-by: Greg Ungerer <gerg@uclinux.org>