config: check for parallel(1), use it for cstyle

Before:
$ time make cstyle
real 0m23.118s
user 0m23.002s
sys 0m0.114s

After:
$ time make cstyle
real 0m4.577s
user 0m31.487s
sys 0m0.699s

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Issue #12899