Update comment to reflect xv6 doesn't have per-cpu segments anymore

Author: kaashoek

trapasm.S

@@ -10,7 +10,7 @@ alltraps:
   pushl %gs
   pushal
 
-  # Set up data and per-cpu segments.
+  # Set up data segments.
   movw $(SEG_KDATA<<3), %ax
   movw %ax, %ds
   movw %ax, %es