libstdc++: Use c++NN_only effective target to tests

libstdc++: Use c++NN_only effective target to tests

Some tests really are only intended for a specific -std mode, so add a
target selector to make that explicit.

Also reorder the dg-do directives to come after the dg-options ones, so
that the target selector in the dg-do directive is applied after the
dg-options that sets the -std option.

libstdc++-v3/ChangeLog:

	* testsuite/20_util/reference_wrapper/83427.cc: Adjust
	effective-target to specific language mode only.
	* testsuite/24_iterators/headers/iterator/range_access_c++11.cc:
	Likewise.
	* testsuite/24_iterators/headers/iterator/range_access_c++14.cc:
	Likewise.
	* testsuite/24_iterators/headers/iterator/synopsis_c++11.cc:
	Likewise.
	* testsuite/24_iterators/headers/iterator/synopsis_c++14.cc:
	Likewise.
	* testsuite/26_numerics/valarray/69116.cc:
	Likewise.
	* testsuite/30_threads/headers/condition_variable/std_c++0x_neg.cc:
	Remove whitespace at end of file.
	* testsuite/30_threads/headers/future/std_c++0x_neg.cc:
	Likewise.