Old amd64 removed and extra upgrade step added

The ‘amd64’ specific parts of kernel architecture have been removed, since x86_64 covers all that.  As a side effect of other changes, John Marino warns that upgrading DragonFly from a version older than 3.4, to a version newer than 3.4, will require an intermediate step of going to 3.4 first.  e.g. If your machine is a DragonFly 3.0 system, you will need to upgrade to 3.4 before moving to, say, 3.6 once it is out.  This won’t matter for some months, since the next release is months off.