In bisect{,-runner}.pl, refactor the code for CPU probing and make jobs.

In bisect{,-runner}.pl, refactor the code for CPU probing and make jobs.

Move the code that attempts various ways to probe for the number of CPUs
from bisect-runner.pl to bisect.pl. Skip the probe entirely if a -j (--jobs)
options is passed to bisect.pl. For --jobs=0 (or -j0) entirely skip adding
-j to the make command line. (For heretical versions of make which don't use
-j for parallelism).

Previously the probe code always ran for each call to bisect-runner.pl,
which is completely redundant if bisect-runner.pl is being called for
argument validation or help text, and inefficient even when building, as the
number of CPUs rarely changes during a bisect run. Additionally there was no
way to avoid a -j in the make command line, which isn't going to fly on
systems where the make utility doesn't have a -j option.