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our help messages getting out of date.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.
8 files changed