cpu/samd21 C++ samr21-xpro support fix

Using the [promoted](https://github.com/RIOT-OS/RIOT/wiki/Board%3A-Samr21-xpro#supported-toolchains) gnu toolchain provides newlib with a `_kill_r()` implementation conflicting with the RIOT one,  so I removed `_kill_r()` from RIOT.
The shipped newlib requires to provide a `_kill()` function when using `g++`, so I added a stub for it.