[$] Rewiring x86 system-call dispatch

Each kernel development cycle includes a vast number of changes that arenot intended to change visible behavior and which, as a result, gounnoticed by most users and developers. One such change in 4.17 is arewiring of how system-call implementations are invoked within the kernel.The change is interesting, though, and provides an opportunity to look atthe macro magic that handles system-call definitions.