m68k: Fix xchg/cmpxchg to fail to link if given an inappropriate pointer

Fix the m68k versions of xchg() and cmpxchg() to fail to link if given an
inappropriately sized pointer rather than BUG()'ing at runtime.

Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Greg Ungerer <gerg@uclinux.org>
cc: linux-m68k@lists.linux-m68k.org