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.
How about just moving the syscall numbers directly into the arch classes in kernel_abi.h and get rid of syscalls.h? They are part of the ABI after all.