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.
I would be fine with doing that, but I think I'll push the SyscallsX86_64 patch to another issue, then.  Getting SyscallsX86_64 to work as a part of the arch classes will require a raft of other things to be added to kernel_abi.h.