s390/s390-64 missing xattr exports

[Roland McGrath]

Aside from the nits you already noted, that looks ok to me to go in as
generic (though I think __BEGIN_DECLS belongs after the #includes).  I
don't have a strong opinion about the header file name.  I would say that
if there is an extant system that this interface is otherwise compatible
with, then use the same header file name.  AFAICT the SGI XFS code is the
only previous thing to be compatible with, and it uses the (silly) name
<attr/xattr.h>.  Despite the silliness of the name, if there is any prior
system anyone ever used to be compatible with and all it takes to be
compatible is a silly header file name, then compatibility seems a more
worthwhile goal than nonsilliness.