libstdc++: Update references to gcc.gnu.org/onlinedocs

libstdc++: Update references to gcc.gnu.org/onlinedocs

libstdc++-v3:
	* doc/xml/manual/abi.xml: Update reference to
	gcc.gnu.org/onlinedocs.
	* doc/xml/manual/concurrency_extensions.xml (interface): Ditto.
	* doc/xml/manual/extensions.xml: Ditto.
	* doc/xml/manual/parallel_mode.xml: Ditto.
	* doc/xml/manual/shared_ptr.xml: Ditto.
	* doc/xml/manual/using_exceptions.xml: Ditto. And change GNU GCC
	to GCC.
	* doc/html/manual/abi.html: Regenerate.
	* doc/html/manual/ext_concurrency_impl.html: Ditto.
	* doc/html/manual/ext_demangling.html: Ditto.
	* doc/html/manual/memory.html: Ditto.
	* doc/html/manual/parallel_mode_design.html: Ditto.
	* doc/html/manual/parallel_mode_using.html: Ditto.
	* doc/html/manual/using_exceptions.html: Ditto.