preserve cs changes from far ctis and iret

_From [bruen...@google.com](https://code.google.com/u/109494838902877177630/) on November 20, 2011 09:58:20_

this was PR 271317

For 64-bit DR in WOW64 (PR 253431) we need to handle application cs changes.
Splitting out from that case.

For common/decode we fail to reproduce the bad-segment fault of a native
lretw (66 cb), and my current SEH64 setup fails to recover from a fault on
only the target of a retw: so for now that test uses a retw. Once we have
faithful cs preservation we can re-enable lretw.

_Original issue: http://code.google.com/p/dynamorio/issues/detail?id=615_