libstdc++: Add explicit -std=gnu++17 option to test

This test has no -std option so when the testsuite is run with
-std=gnu++20 or later, this test will use that. The recent addition of
no_unique_address will cause it to FAIL, because that's a reserved word
after C++17. Add an explicit option, so that this test alays uses
exactly C++17.

libstdc++-v3/ChangeLog:

	* testsuite/17_intro/headers/c++2017/all_attributes.cc: Add
	-std=gnu++17 option.

(cherry picked from commit 45fb3d45a4)

libstdc++-v3/testsuite/17_intro/headers/c++2017/all_attributes.cc

@@ -15,6 +15,7 @@
 // with this library; see the file COPYING3.  If not see
 // <http://www.gnu.org/licenses/>.
 
+// { dg-options "-std=gnu++17" }
 // { dg-do compile { target c++17 } }
 
 // Ensure the library only uses the __name__ form for attributes.