[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>

arch/s390/lib/Makefile

@@ -5,6 +5,6 @@
 EXTRA_AFLAGS := -traditional
 
 lib-y += delay.o string.o uaccess_std.o uaccess_pt.o qrnnd.o
-lib-$(CONFIG_32BIT) += div64.o
+obj-$(CONFIG_32BIT) += div64.o
 lib-$(CONFIG_64BIT) += uaccess_mvcos.o
 lib-$(CONFIG_SMP) += spinlock.o