file-systems: Support the 'no-atime' flag.

* guix/build/syscalls.scm (MS_NOATIME): New variable.
* gnu/build/file-systems.scm (mount-flags->bit-mask): Support it.
* doc/guix.texi (File Systems): Document it and add cross-references to
the relevant documentation.

Co-authored-by: Ludovic Courtès <ludo@gnu.org>