arch/x86/ia32: move IA32 thread state to _thread_arch

There are not enough bits in k_thread.thread_state with SMP enabled,
and the field is (should be) private to the scheduler, anyway. So
move state bits to the _thread_arch where they belong.

While we're at it, refactor some offset data w/r/t _thread_arch
because it can be shared between 32- and 64-bit subarches.

Signed-off-by: Charles E. Youse <charles.youse@intel.com>