Use sudo if /proc/cpuinfo isn't readable in the start-tor-browser script

On Linux, the start-tor-browser script checks for SSE2 support by grepping /proc/cpuinfo. On some systems, /proc/cpuinfo isn't readable to ordinary users (Whonix may implement this) so it will fail with an error saying "Tor Browser requires a CPU with SSE2 support.".

It would be better if the script checked if the file is not readable and if it isn't, it tries to grep it with sudo --noninteractive so people can add sudoers exceptions for the Tor Browser.