s390/cputime: fix 31-bit compile

git commit 8461b63c
"s390: translate cputime magic constants to macros"
introduce a built error for 31-bit:

  kernel/built-in.o: In function `posix_cpu_timer_set':
  posix-cpu-timers.c:(.text+0x2a8cc): undefined reference to `__udivdi3'

The original code is actually broken for 31-bit and has been
corrected by the above commit by forcing the compiler to use
64-bit arithmetic through the CPUTIME_PER_USEC define.

To fix the compile error replace the 64-bit division with
a call to __div().

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>