incorrect/useless call to printf() in _exit()

[WestfW]

in cores/arduino/syscalls_sam3.c, the implementation of _exit() includes:

extern void _exit( int status )
{
    printf( "Exiting with status %d.\n", status ) ;

    for ( ; ; ) ;
}

This causes something like 12k of printf and stdio code to be included in every sketch, and probably doesn't do anything useful since there is no guarantee that the UART has been initialized at all (even assuming that printf() ends up using the skeleton _write() code that is also there...)

[facchinm]

Thanks for spotting, if you could add a PR I'll be glad to merge it (and release an updated SAM core after a looong time)