check nobody user/group validity only when not cross compiling

check nobody user/group validity only when not cross compiling

Using `getent' and `id' command in case of cross compiling does not
make much sense. This is because it is the host files that are checked.

Besides, in some restricted cross compilation environment, these two
command may not even be available. This is to avoid host comtamination.

So we should only check the validity using getent and id when not
cross compiling.