QOM CPUState refactorings / X86CPU

* Fix for X86CPU model field of qemu32/qemu64 CPU models
* Bug fix for longjmp on FreeBSD
* Removal of unused function
* Confinement of clone syscall infrastructure to linux-user

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# By Andreas Färber (2) and others
# Via Andreas Färber
* afaerber/tags/qom-cpu-for-anthony:
  cpu: Drop cpu_model_str from CPU_COMMON
  cpu: Move cpu_copy() into linux-user
  cputlb: Remove dead function tlb_update_dirty()
  cpu-exec: Also reload CPUClass *cc after longjmp return in cpu_exec()
  target-i386: Set model=6 on qemu64 & qemu32 CPU models