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.
Making `base_arch<supported_arch arch>` inherit from `Syscalls<arch>` is pretty straightforward.  I think trying to make `x86_arch` inherit from `Syscalls<x86_arch>` might run into circularity problems, unless you can specialize `Syscalls` on a class that isn't yet defined...