Fix demangle style usage info

Extract allowed styles from libiberty, so we don't have to worry about
our help messages getting out of date.  The function probably belongs
in libiberty/cplus-dem.c but it can be here for a while to iron out
bugs.

	PR 28581
	* demanguse.c: New file.
	* demanguse.h: New file.
	* nm.c (usage): Break up output.  Use display_demangler_styles.
	* objdump.c (usage): Use display_demangler_styles.
	* readelf.c (usage): Likewise.
	* Makefile.am: Add demanguse.c and demanguse.h.
	* Makefile.in: Regenerate.
	* po/POTFILESin: Regenerate.

binutils/demanguse.c

@@ -0,0 +1,54 @@
+/* demanguse.c -- libiberty demangler usage
+   Copyright (C) 2021 Free Software Foundation, Inc.
+
+   This file is part of GNU Binutils.
+
+   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, write to the Free Software
+   Foundation, Inc., 51 Franklin Street - Fifth Floor, Boston, MA
+   02110-1301, USA.  */
+
+#include <stdio.h>
+#include <string.h>
+#include "demangle.h"
+#include "demanguse.h"
+
+/* Print the list of demangling styles to STREAM.  A one line MSG is
+   printed before the styles.  Output is limited to 80 columns, with
+   continuation lines being indented by leading spaces in MSG.  */
+
+void
+display_demangler_styles (FILE *stream, const char *msg)
+{
+  const struct demangler_engine *info = libiberty_demanglers;
+  int col;
+  int lead_spaces = 0;
+  const char *cont = "";
+
+  while (msg[lead_spaces] == ' ')
+    ++lead_spaces;
+  col = fprintf (stream, "%s", msg);
+  while (info->demangling_style_name)
+    {
+      if (col + strlen (info->demangling_style_name) >= 75)
+	{
+	  fprintf (stream, "%.1s\n", cont);
+	  col = fprintf (stream, "%.*s", lead_spaces, msg);
+	  cont = "";
+	}
+      col += fprintf (stream, "%s\"%s\"", cont, info->demangling_style_name);
+      cont = ", ";
+      ++info;
+    }
+  fprintf (stream, "\n");
+}