libstdc++: Require c++98_only effective target for some tests
These tests verify that including C++11 headers fails to compile in
C++98 mode. They use { dg-options "-std=gnu++98" } so that they are
explicitly run in C++98 mode. This change also adds a target selector so
that the tests will be skipped even if the dg-options directive is
filtered out or overridden. This is in preparation for a desired future
change where tests do not use -std options, so that they can be tested
with e.g. --target_board=unix\"{-std=gnu++17,-std=gnu++20}\"
In some cases the dg-options and dg-do directives need to be reordered,
so that the -std=gnu++98 option is already added to the options before
the target selector is checked.