libstdc++: check for openat

libstdc++: check for openat

rtems6.0 has fdopendir, and fcntl.h defines AT_FDCWD and declares
openat, but there's no openat in libc.  Adjust dir-common.h to not
assume ::openat just because of AT_FDCWD.

for  libstdc++-v3/ChangeLog

	* acinclude.m4 (GLIBCXX_CHECK_FILESYSTEM_DEPS): Check for
	openat.
	* configure, config.h.in: Rebuilt.
	* src/filesystem/dir-common.h (openat): Use ::openat if
	_GLIBCXX_HAVE_OPENAT.
	* src/filesystem/dir.cc (dir_and_pathname): Use dirfd if
	_GLIBCXX_HAVE_OPENAT.