Make sure only Linux users with SSE2 support are trying to use Tor Browser based on ESR60

There might still be Linux users without SSE2 support out there but since Firefox 53 the SSE2 requirement for Linux is active, too, see: https://bugzilla.mozilla.org/show_bug.cgi?id=1274196.

We should

a) make sure Linux users without SSE2 are not updating to the new Tor Browser (we have minSupportedInstructionSet: SSE2 for Windows already which might be enough here)

and

b) amend our start script to check for SSE2 support.

For more information and the Windows context, see: #19316 (moved).