cpu: added `extern "C"` to headers

This PR adds `extern "C"` to the header files in ./RIOT/cpu/* folders.
These two blocks should be merged into one. (<sys/select.h> is a system header)
If not, 
```
#ifdef __cplusplus
}
#endif
```
should be placed before ``` #ifdef MODULE_UART0 ``` and 
```
#ifdef __cplusplus
extern "C" {
#endif
```
should be after ``` #endif ``` (line 143).
The choice is yours ;-)