elfedit: Add --output-abiversion option to update ABIVERSION

elfedit: Add --output-abiversion option to update ABIVERSION

	* NEWS: Mention --output-abiversion.
	* elfedit.c (input_elf_abiversion): New.
	(output_elf_abiversion): Likewise.
	(update_elf_header): Update EI_ABIVERSION.
	(command_line_switch): Add OPTION_INPUT_ABIVERSION and
	OPTION_OUTPUT_ABIVERSION.
	(options): Add --input-abiversion and --output-abiversion.
	(usage): Likewise.
	(main): Handle --input-abiversion and --output-abiversion.
	* doc/binutils.texi: Document --input-abiversion and
	--output-abiversion.
	* testsuite/binutils-all/elfedit.exp: Run elfedit-6.
	* testsuite/binutils-all/elfedit-6.d: New file.