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.
This is no longer needed, so you can remove this part of the comment as well as the redundant SYSCALL_UNDEFINED_X86_64()s.