cpu/stm32f3: c++: initial support for c++, added _kill (weak) and weak attribute for _kill_r
boards/stm32f3discovery: provided c++ feature (cpp)
Merge pull request #1989 from BytesGalore/EasyCPPMixing_stm32f3discovery_support_fix

cpu/stm32f3 C++ stm32f3discovery support fix