libstdc++: Require c++98_only effective target for a test

libstdc++: Require c++98_only effective target for a test

This test checks a conversion which only exists in C++98 and won't
compile since C++11. It uses { dg-options "-std=gnu++98" } so that it is
explicitly run in C++98 mode. This change also adds a target selector so
that the test will be skipped if the dg-options directive is filtered
out or overridden.

libstdc++-v3/ChangeLog:

	* testsuite/27_io/basic_ios/conv/voidptr.cc: Add c++98_only
	target selector.