cpu/sam3x8e C++ arduino due support fix

Using `g++` links to the newlib implementation of `_kill_r` which conflicts the RIOT own implementation in `syscalls.c`, so I removed RIOT `_kill_r` as it has no real implementation/purpose.
Additionally c++ requires to be provided with a `_kill()` function, so I added a stub for it (currently).
Please comment this function (in some way...).