Use dg-additional-options in libstdc++ tests

Use dg-additional-options in libstdc++ tests

	* testsuite/17_intro/headers/c++2011/stdc++.cc: Change target-specific
	dg-options to dg-additional-options so that default options are used.
	* testsuite/17_intro/headers/c++2011/stdc++_multiple_inclusion.cc:
	Likewise.
	* testsuite/17_intro/headers/c++2014/stdc++.cc: Likewise.
	* testsuite/17_intro/headers/c++2014/stdc++_multiple_inclusion.cc:
	Likewise.
	* testsuite/29_atomics/atomic_flag/test_and_set/explicit-hle.cc:
	Use dg-additional-options instead of repeating the common options.

From-SVN: r238835