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.
Actually I wonder if we could make the Arch classes inherit from Syscalls<Arch>. Then we could write just Arch::readv etc which looks nicer than Syscalls<Arch>::readv.