libstdc++: Improvements to standard error category objects (part deux)

libstdc++: Improvements to standard error category objects (part deux)

In r12-3860 the error categories in <system_error> were made final and
immortal, but I missed the categories for <future> and <ios>. This makes
the same changes to those.

libstdc++-v3/ChangeLog:

	* src/c++11/cxx11-ios_failure.cc (io_error_category): Define
	class and virtual functions as 'final'.
	(io_category_instance): Use constinit union to make the object
	immortal.
	* src/c++11/future.cc (future_error_category): Define class and
	virtual functions as 'final'.
	(future_category_instance): Use constinit union.