scripts: coccicheck: Change default condition for parallelism

Currently, Coccinelle uses at most one thread per core by default in
machines with more than 2 hyperthreads. However, for systems with only 4
hyperthreads, this does not improve performance.

Modify coccicheck to use all available threads in machines with
upto 4 hyperthreads.

Signed-off-by: Sumera Priyadarsini <sylphrenadin@gmail.com>
Signed-off-by: Julia Lawall <Julia.Lawall@inria.fr>