System statistics are not implemented on Mac

[mellowagain]

System statistics have been implemented on every platform except Darwin, where every method will throw a std::runtime_error when invoked. As Shiro invokes the native::system_statistics::init method on startup, this renders Shiro unable to start on Mac.

The file that requires implementation can be found here: https://github.com/Marc3842h/shiro/blob/master/src/native/mac/system_statistics_mac.cc