x86: never set i386_cpu_flags' "unused" field

x86: never set i386_cpu_flags' "unused" field

Setting this field risks cpu_flags_all_zero() mistakenly returning
"false" when the object passed in was e.g. the result of ANDing together
two objects which had the bit set, or ANDNing together an object with
the field set and one with the field clear.

While there also avoid setting CpuNo64: Like Cpu64 this is driven
differently anyway and hence shouldn't be set anywhere by default.

Note that the moving of the two items in i386-gen.c's cpu_flags[] is
only for documentation purposes (and slight reducing of overhead), as
the fields are sorted anyway upon program start.