S390: Don't test nanoseconds in io/tst-stat.c

Both new tests io/tst-stat and io/tst-stat-lfs (_FILE_OFFSET_BITS=64)
are comparing the nanosecond fields with the statx result.  Unfortunately
on s390(31bit) those fields are always zero if old KABI with non-LFS
support is used.  With _FILE_OFFSET_BITS=64 stat is using statx internally.

As suggested by Adhemerval this patch disables the nanosecond check for
s390(31bit).
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>