cpu: added `extern "C"` to headers

This PR adds `extern "C"` to the header files in ./RIOT/cpu/* folders.
Only one extern "C" is needed with 
``` C
#ifdef __cplusplus
extern "C" {
#endif
``` 
at line 172 and 
``` C
#ifdef __cplusplus
}
#endif
```
at line 666.
Or we can just leave this file intact. :-P