format-cpp: run clang-format in parallel

format-cpp: run clang-format in parallel

Use the -P option of GNU xargs to run multiple instances of clang-format
in parallel, which speeds up the execution quite a bit (depending on the
number of cores, of course).

Inspired by this babeltrace commit:

  http://git.efficios.com/?p=babeltrace.git;a=commit;h=66c3bce11973e6e96a3791c378a9e5f98ddaa280

Change-Id: I201535244ef4c3614dfd742ae6f1c427994e6147
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>