re PR libstdc++/54314 (undefined references to 'construction vtable for std::ostream-in-std::basic_ostringstream<char, std::char_traits<char>, std::allocator<char> >')

	PR libstdc++/54314
gcc/
	* varasm.c (default_assemble_visibility): Don't warn about
	visibility on artificial decls.
gcc/cp/
	* class.c (build_ctor_vtbl_group): Give construction vtables
	hidden visibility.
libstdc++-v3/
	* config/abi/pre/gnu.ver: Don't export construction vtables.
	* config/abi/pre/gnu-versioned-namespace.ver: Likewise.

From-SVN: r195550