chainlint.pl: latch CPU count directly reported by /proc/cpuinfo

chainlint.pl: latch CPU count directly reported by /proc/cpuinfo

On Linux, ncores() computes the number of CPUs by counting the
"processor" or "CPU" lines emitted by /proc/cpuinfo. However, on some
platforms, /proc/cpuinfo does not enumerate the CPUs at all, but
instead merely mentions the total number of CPUs. In such cases, pluck
the CPU count directly from the /proc/cpuinfo line which reports the
number of active CPUs. (In particular, check for "cpus active: NN" and
"ncpus active: NN" since both variants have been seen in the
wild[1,2].)

[1]: https://lore.kernel.org/git/503a99f3511559722a3eeef15d31027dfe617fa1.camel@physik.fu-berlin.de/
[2]: https://lore.kernel.org/git/7acbd5c6c68bd7ba020e2d1cc457a8954fd6edf4.camel@physik.fu-berlin.de/

Reported-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>