lib/systems: fix kernelArch for x86_64

IA64 (Itanium) is something completely different and certainly not
what we want! x86_64 code lives in arch/x86 just like "classic" x86.

lib/systems/default.nix

···
         if final.isAarch32 then "arm"
         else if final.isAarch64 then "arm64"
         else if final.isx86_32 then "x86"
-        else if final.isx86_64 then "ia64"
+        else if final.isx86_64 then "x86"
         else if final.isMips then "mips"
         else final.parsed.cpu.name;