chainlint.pl: fix /proc/cpuinfo regexp

chainlint.pl: fix /proc/cpuinfo regexp

29fb2ec3 (chainlint.pl: validate test scripts in parallel,
2022-09-01) introduced a function that gets the number of cores from
/proc/cpuinfo on some systems, notably linux.

The regexp it uses (^processor\s*:) fails to match the desired lines in
the s390x architecture, where they look like this:

    processor 0: version = FF, identification = 148F67, machine = 2964

As a result, on s390x that function returns 0 as the number of cores,
and the chainlint.pl script exits without doing anything.

Signed-off-by: Andreas Hasenack <andreas.hasenack@canonical.com>
Acked-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>