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 think being explicit about exactly what syscalls are undefined is better than silently letting undefinedness "leak through" multiple syscall definitions.  I realize the numbering scheme "leaks through", but I think numbering is different than undefinedness.