Downstream patch to keep ABI stable.
Bring back a dummy `struct r_debug` member in `GL (dl_ns)`, to preserve
`_rtld_global` layout.

diff --git a/sysdeps/generic/ldsodefs.h b/sysdeps/generic/ldsodefs.h
index e09edb01da3b5b90..3d9b90a22bfa6a7d 100644
--- a/sysdeps/generic/ldsodefs.h
+++ b/sysdeps/generic/ldsodefs.h
@@ -375,6 +375,8 @@ struct rtld_global
       size_t n_elements;
       void (*free) (void *);
     } _ns_unique_sym_table;
+    /* Dummy structure to keep the ABI stable.  */
+    struct r_debug _ns_debug_unused;
   } _dl_ns[DL_NNS];
   /* One higher than index of last used namespace.  */
   EXTERN size_t _dl_nns;