s390/cputime: provide archicture specific cputime_to_nsecs

The generic cputime_to_nsecs function first converts the cputime
to micro-seconds and then multiplies the result with 1000. This
looses some bits of accuracy, provide our own version of
cputime_to_nsecs that does not loose precision.

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