cpu: added `extern "C"` to headers

This PR adds `extern "C"` to the header files in ./RIOT/cpu/* folders.
Yeah, we can do that because it has ``` extern "C" ``` block already and ``` #include <cmsis_ccs.h> ``` ``` #include <cmsis_iar.h> ``` look like system headers or compiler-specific headers to me.
> The same would apply for the next header (cpu/cortex-4_common/include/core_cmFunc.h)

Maybe we just need one extern "C" block for this one.