Add --decompress option to readelf to decompress sections before they are dumped.

Add --decompress option to readelf to decompress sections before they are dumped.

bin	* readelf.c (options): Add "decompress".
	(usage): Mention -z/--decompress.
	(parse_args): Handle -z.
	(uncompress_section_contents): Move to earlier in the file.
	(dump_section_as_strings): If requested, decompress the section
	before dumping.
	(dump_section_as_bytes): Likewise.
	* doc/binutils.texi: Document the new option.

tests	* binutils-all/z.s: New test.  Checks the --decompress option to
	readelf.
	* binutils-all/readelf.exp: Run the test.
	* binutils-all/readelf.z: Expected output from readelf.