Each pch file is updated independently from the others,
giving an opportunity to execute the update logic in parallel.
The script uses xargs to run the update script in parallel.

The total execution on my particular rig was reduced from
~420 minutes to under 75 mins, using 24 threads on Evo 850 SSD.

Change-Id: Ifeb3947e756325756eb786e3025155481da1ea41
Reviewed-on: https://gerrit.libreoffice.org/14802Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>