Fix building and testing dwarf debug section compression feature when zlib is not...

Fix building and testing dwarf debug section compression feature when zlib is not available.

	PR gas/18087
gas/test	* gas/i386/dw2-compress-1.d: Allow the test to pass regardless of
	whether the .debug_info section was compressed on not.

bfd	* compress.c (bfd_compress_section_contents): Do not define this
	function if it is not used.