* testsuite/lib/libstdc++.exp: Set additional_prunes.
	(libstdc++-dg-test): Don't prune here.
	* testsuite/lib/prune.exp (dg-prune-output): New.
	(libstdc++-dg-prune): Rename from prune_g++_output.
	Add a bunch of prunes from gcc's prune.exp.

From-SVN: r160812