[S390]: Fix build on 31-bit.

Allow s390 to properly override the generic
__div64_32() implementation by:

1) Using obj-y for div64.o in s390's makefile instead
   of lib-y

2) Adding the weak attribute to the generic implementation.

Signed-off-by: David S. Miller <davem@davemloft.net>

lib/div64.c

@@ -23,7 +23,7 @@
 /* Not needed on 64bit architectures */
 #if BITS_PER_LONG == 32
 
-uint32_t __div64_32(uint64_t *n, uint32_t base)
+uint32_t __attribute__((weak)) __div64_32(uint64_t *n, uint32_t base)
 {
 	uint64_t rem = *n;
 	uint64_t b = base;