iot-lab_m3: C++ support

This PR will provide C++ support for iot-lab_M3.
Problem with ```_kill_r``` is resolved by adding ``` __attribute__ ((weak)) ```, thus, if ```_kill_r``` was provided by compiler then we'll use the one provided by compiler instead and ```_kill``` is also implemented with weak attribute.
nice one. The weak attribute prevents the function to be linked if provided elsewhere, e.g. newlib.
I will steal this to the other c++ support PRs ;)