coccicheck: span checks across CPUs

This adds parallelism by default to the "coccicheck" target using
spatch's "-max" and "-index" arguments.

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Nicolas Palix <nicolas.palix@imag.fr>
Signed-off-by: Michal Marek <mmarek@suse.cz>