Fix undefined references in libstdc++fs.a

Fix undefined references in libstdc++fs.a

The recent patch for PR 83306 removed the fs_err_concat functions that
were used by the experimental::filesystem::filesystem_error class as
well. This fixes it by doing the string generation directly in
filesystem_error::_M_gen_what() instead of using the removed function.

	PR libstdc++/83306
	* src/filesystem/path.cc (filesystem_error::_M_gen_what()): Create
	string directly, instead of calling fs_err_concat.

From-SVN: r266569