libstdc++: Improvements to standard error category objects

This ensures that the objects returned by std::generic_category() and
std::system_category() are initialized before any code starts executing,
and are not destroyed at the end of the program. This means it is always
safe to access them, even during startup and termination. See LWG 2992
and P1195R0 for further discussion of this.

Additionally, make the types of those objects final, which might
potentially allow additional devirtualization opportunities. The types
are not visible to users, so there is no way they can derive from them,
so making them final has no semantic change.

Finally, add overrides for equivalent(int, const error_condition&) to
those types, to avoid the second virtual call that would be performed by
the base class definition of the function. Because we know what
default_error_condition(int) does for the derived type, we don't need to
make a virtual call.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>

libstdc++-v3/ChangeLog:

	* src/c++11/system_error.cc (generic_error_category): Define
	class and virtual functions as 'final'.
	(generic_error_category::equivalent(int, const error_condition&)):
	Override.
	(system_error_category): Define class and virtual functions as
	'final'.
	(system_error_category::equivalent(int, const error_condition&)):
	Override.
	(generic_category_instance, system_category_instance): Use
	constinit union to make the objects immortal.

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