tools/format-cpp: run `xargs` with `-P$(nproc)` (format in parallel)

tools/format-cpp: run `xargs` with `-P$(nproc)` (format in parallel)

This saves many seconds per run.

Note that some implementations of xargs might not accept the `-P`
option, therefore `tools/format-cpp` now requires something like
GNU xargs.

`nproc` is also a GNU (Coreutils) thing.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: Ie7113dae32cff506b716c3b590bbe58eac5f1586
Reviewed-on: https://review.lttng.org/c/babeltrace/+/9950
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-by: Michael Jeanson <mjeanson@efficios.com>