libstdc++: Fix test that fails with -ffreestanding

The -ffreestanding option disables Debug Mode, forcibly #undef'ing
_GLIBCXX_DEBUG. This means that the dangling checks in std::pair are
disabled for -ffreestanding in C++17 and earlier, because they depend on
_GLIBCXX_DEBUG. Adjust the target specifiers for the errors currently
matching c++17_down so they also require the hosted effective target.

libstdc++-v3/ChangeLog:

	* testsuite/20_util/pair/dangling_ref.cc: Add hosted effective
	target for specifiers using c++17_down.