sim: start unifying portability shims

There are some functions that gnulib does not yet provide fallbacks
for, so start a common file of our own for holding existing stubs.

sim/common/portability.c

@@ -0,0 +1,67 @@
+/* Portability shims for missing OS support.
+   Copyright (C) 2021 Free Software Foundation, Inc.
+   Contributed by Mike Frysinger.
+
+This file is part of the GNU Simulators.
+
+This program is free software; you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation; either version 3 of the License, or
+(at your option) any later version.
+
+This program is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+/* This must come before any other includes.  */
+#include "defs.h"
+
+#include <unistd.h>
+
+#include "portability.h"
+
+#ifndef HAVE_GETEGID
+int getegid(void)
+{
+  return 0;
+}
+#endif
+
+#ifndef HAVE_GETEUID
+int geteuid(void)
+{
+  return 0;
+}
+#endif
+
+#ifndef HAVE_GETGID
+int getgid(void)
+{
+  return 0;
+}
+#endif
+
+#ifndef HAVE_GETUID
+int getuid(void)
+{
+  return 0;
+}
+#endif
+
+#ifndef HAVE_SETGID
+int setgid(int gid)
+{
+  return -1;
+}
+#endif
+
+#ifndef HAVE_SETUID
+int setuid(int uid)
+{
+  return -1;
+}
+#endif