Started with GDB 10.1, the attach to a process was crashing
because the_procfs_target variable had not been initialized
with a valid process_stratum_target structure. To workaround
the issue, we extract a valid stratum from the current inferior.

Will be sent to upstream.

diff -wpruN '--exclude=*.orig' a~/gdb/procfs.c a/gdb/procfs.c
--- a~/gdb/procfs.c	1970-01-01 00:00:00
+++ a/gdb/procfs.c	1970-01-01 00:00:00
@@ -1863,7 +1863,7 @@ do_attach (ptid_t ptid)
 
   /* Add it to gdb's thread list.  */
   ptid = ptid_t (pi->pid, lwpid, 0);
-  thread_info *thr = add_thread (&the_procfs_target, ptid);
+  thread_info *thr = add_thread (inf->process_target(), ptid);
   switch_to_thread (thr);
 }