m68k: __cpu_simple_unlock audit.

- Use `cc' clobbers in asm volatile because they touch the condition
  codes.

- Use `memory' clobbers in asm volatile so the compiler doesn't move
  up loads and stores in the critical section at _other_ addresses
  than the lock so they happen before __cpu_simple_lock or
  __cpu_simple_lock_try.

- Not sure if we have any (or if there even are any?) multicore m68k
  systems out there, but __cpu_simple_unlock needs __insn_barrier
  either way so the compiler doesn't delay loads and stores prior to
  __cpu_simple_unlock so they happen after it.

(riastradh)