y2038: Use a common definition for stat

Instead of replicate the same definitions from struct_stat_time64.h
on the multiple struct_stat.h, use a common header which is included
when required (struct_stat_time64_helper.h).  The 64-bit time support
is added only for LFS support.

The __USE_TIME_BITS64 is not defined internally yet, although the
internal header is used when building the 64-bit stat implementations.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>
Tested-by: Carlos O'Donell <carlos@redhat.com>