log: Fix random segv when logging at static initialisation time.

Change-Id: Ifd2e53598610b0bf5c2d08bd165ed7857d541233
Reviewed-on: http://gerrit.aldebaran.lan/22702
Reviewed-by: mnottale <mnottale@aldebaran-robotics.com>
Tested-by: mnottale <mnottale@aldebaran-robotics.com>
os::cputime(): New.

Change-Id: Id82a6ff8d0e94d3476959f8a4ab55e838ce31747
Reviewed-on: http://gerrit.aldebaran.lan/22370
Tested-by: gerrit
Reviewed-by: mnottale <mnottale@aldebaran-robotics.com>
Tested-by: mnottale <mnottale@aldebaran-robotics.com>
Fix compilation on MacOS (RUSAGE_THREAD doesn't exist on mac)

Change-Id: I268c9fe1dfba6dc1b6233ef65875641dad25c00b
Reviewed-on: http://gerrit.aldebaran.lan/22757
Reviewed-by: hcuche <hcuche@aldebaran-robotics.com>
Reviewed-by: mnottale <mnottale@aldebaran-robotics.com>
Tested-by: hcuche <hcuche@aldebaran-robotics.com>