x86: corrections to CPU attribute/flags splitting

x86: corrections to CPU attribute/flags splitting

There are a number of issues with 734dfd1cc966 ("x86: pack CPU flags in
opcode table"):
- the condition when two array slots need writing wasn't correct (with
  enough new Cpu* added an out of bounds array access would validly have
  been complained about by the compiler),
- table generation didn't take into account CpuAttrUnused and CpuUnused
  being independent, and hence there not always (not) being an "unused"
  bitfield member in both structures,
- cpu_flags_from_attr() wasn't ready for use on big-endian hosts,
- there were two style violations.