cpu/samd12: c++: initial support for c++, added _kill (weak) and weak attribute for _kill_r
boards/samr21-xpro: provided c++ feature (cpp)
Merge pull request #1994 from BytesGalore/EasyCPPMixing_samr21_xpro_support_fix

cpu/samd21 C++ samr21-xpro support fix