libstdc++: Use std::construct_at in net::ip::address

libstdc++: Use std::construct_at in net::ip::address

Using placement-new isn't valid in constant expressions, so this
replaces it with std::construct_at (via the std::_Construct function
that is usable before C++20).

libstdc++-v3/ChangeLog:

	* include/experimental/internet (address): Use std::_Construct
	to initialize union members.