Documentation: describe how to add a system call

Add a document describing the process of adding a new system call,
including the need for a flags argument for future compatibility, and
covering 32-bit/64-bit concerns (albeit in an x86-centric way).

Signed-off-by: David Drysdale <drysdale@google.com>
Reviewed-by: Michael Kerrisk <mtk.manpages@gmail.com>
Reviewed-by: Eric B Munson <emunson@akamai.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>