libstdc++: Fix directory iterator build for newlib

libstdc++: Fix directory iterator build for newlib

When building for newlib HAVE_OPENAT and HAVE_UNLINKAT are (sometimes?)
defined, but <fcntl.h> is only included when HAVE_DIRENT_H is defined.
Since directory iterators are completely useless without <dirent.h>,
just override the HAVE_OPENAT and HAVE_UNLINKAT detection when we don't
have <dirent.h>.

libstdc++-v3/ChangeLog:

	* src/filesystem/dir-common.h (_GLIBCXX_HAVE_DIRFD): Undefine
	when <dirent.h> is not available.
	(_GLIBCXX_HAVE_UNLINKAT):  Likewise.