m68k: use asm-generic cmpxchg_local()

Use the asm-generic version of the cmpxchg_local() macro. Although not
all target types use asm-generic/cmpxchg.h, for those that do the
local cmpxchg_local() is the same as the asm-generic/cmpxchg.h one.
So no need to define the local one.

Signed-off-by: Greg Ungerer <gerg@linux-m68k.org>