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our help messages getting out of date.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.h

@@ -0,0 +1,34 @@
+/* demanguse.h -- 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.  */
+
+#ifndef DEMANGUSE_H
+#define DEMANGUSE_H
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+void display_demangler_styles (FILE *, const char *);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* DEMANGUSE_H */