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: NDavid Drysdale <drysdale@google.com>
Reviewed-by: NMichael Kerrisk <mtk.manpages@gmail.com>
Reviewed-by: NEric B Munson <emunson@akamai.com>
Reviewed-by: NKees Cook <keescook@chromium.org>
Reviewed-by: NRandy Dunlap <rdunlap@infradead.org>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>