Honor dd's noatime flag if possible, even if not supported on build fs

Honor dd's noatime flag if possible, even if not supported on build fs
* doc/coreutils.texi (dd invocation): Warn that noatime might not be
reliable.
* src/dd.c (flags, usage): Look at O_NOATIME, not
HAVE_WORKING_O_NOATIME, to decide whether to support the noatime
flag, so that dd attempts O_NOATIME even if the build file system
does not support it.  Problem reported by Jim Meyering today in
bug-coreutils.
* tests/dd/misc: Generate a warning, not a failure, if noatime
exists but fails.