re PR libstdc++/25524 (libstdc++ headers should go in multilib directories)

	PR libstdc++/25524
	* cppdefault.h (struct default_include): Add multilib flag.
	* cppdefault.c (cpp_include_defaults): Set it.
	* c.opt (-imultilib): New option.
	* c-opts.c (imultilib): New.
	(c_common_handle_option): Handle -imultilib.
	(c_common_post_options): Likewise.
	* c-incpath.c (add_standard_paths, register_include_chains):
	Likewise.
	* c-incpath.h (register_include_chains): Add extra parameter.
	* gcc.c (do_spec_1): Generate -imultilib option.
	(The Specs Language): Update %I description.
	(process_command): Update copyright notice.
	* doc/cppopts.texi (-imultilib): Document.
	* doc/invoke.texi (-imultilib): Include in option summary.
	(%I): Update specs documentation.

libstdc++-v3:
	* include/Makefile.am: Install host-specific headers in multilib
	subdirectory.
	* include/Makefile.in: Regenerate.

From-SVN: r110037