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.
Ah, so specialize on `x86_arch` instead?  That will probably work better once more things get templated.  I'll do that.