X86-64 syscall numbers

This series starts implementing what I think @rocallahan was describing in his comments on #1206.  (Thanks for fixing up bustage as a result of that, btw.)  Ideally the commit messages for the individual patches should describe what's going on.
Very nice.

But then we have to forward-declare the list of all `*_arch` classes in `syscalls.h`; at that point, just having `Syscalls<supported_arch>` and letting `base_arch` inherit from that seems like more of a win.