[PATCH] Fix problem introduced by do_div() patch

[PATCH] Fix problem introduced by do_div() patch

 - export the __div64_32 symbol for modules;

 - add likely() to the fast path (divisor>>32 == 0);

 - add __attribute__((pure)) to __div64_32() prototype so
   the compiler knows global memory isn't clobbered;

 - avoid building __div64_32() on 64bit architectures.