Fix demangle style usage info

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our help messages getting out of date.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.