Define PS_USE_CLOBBER_ARGV on GNU/Hurd.

This is a backpatch of 32b23664 to v14 and v15.

* Remove ancient test for __hurd__, which intended to activate
  PS_USE_CHANGE_ARGV in these branches but turned out not to be
  defined on modern systems.

* Add new test for__GNU__ to activate PS_USE_CLOBBER_ARGV, like the
  newer branches.

Author: Michael Banck <mbanck@debian.org>
Discussion: https://postgr.es/m/CA%2BhUKGJMNGUAqf27WbckYFrM-Mavy0RKJvocfJU%3DJ2XcAZyv%2Bw%40mail.gmail.com
Backpatch-through: 14-15

src/backend/utils/misc/ps_status.c

@@ -73,9 +73,9 @@ bool		update_process_title = true;
 #define PS_USE_PSTAT
 #elif defined(HAVE_PS_STRINGS)
 #define PS_USE_PS_STRINGS
-#elif (defined(BSD) || defined(__hurd__)) && !defined(__darwin__)
+#elif defined(BSD) && !defined(__darwin__)
 #define PS_USE_CHANGE_ARGV
-#elif defined(__linux__) || defined(_AIX) || defined(__sgi) || (defined(sun) && !defined(BSD)) || defined(__svr5__) || defined(__darwin__)
+#elif defined(__linux__) || defined(_AIX) || defined(__sgi) || (defined(sun) && !defined(BSD)) || defined(__svr5__) || defined(__darwin__) || defined(__GNU__)
 #define PS_USE_CLOBBER_ARGV
 #elif defined(WIN32)
 #define PS_USE_WIN32
@@ -85,7 +85,7 @@ bool		update_process_title = true;
 
 
 /* Different systems want the buffer padded differently */
-#if defined(_AIX) || defined(__linux__) || defined(__darwin__)
+#if defined(_AIX) || defined(__linux__) || defined(__darwin__) || defined(__GNU__)
 #define PS_PADDING '\0'
 #else
 #define PS_PADDING ' '