remove -gz=zlib-gnu option value

The option value is legacy and probably not used at all,
thus ignore it.

gcc/ChangeLog:

	* configure: Regenerate.
	* configure.ac: Simplify to gcc_cv_ld_compress_debug={0,1}
	and gcc_cv_as_compress_debug={0,1}.
	* doc/invoke.texi: Document the removal.
	* gcc.cc (LINK_COMPRESS_DEBUG_SPEC): Simplify and ignore
	  zlib-gnu.
	(ASM_COMPRESS_DEBUG_SPEC): Likewise.

Co-Authored-By: Fangrui Song <i@maskray.me>