[PATCH] Fix -mthumb resetting of target cpu

[Renato Golin]

  I really don't like these functions, but I won't complain about this change so much. I thought about some string replacement along the lines of /^thumb/arm/ and keep the switch as it is, but I'm not sure how to do that properly on StringRefs.

  I agree with Tim about the FIXMEs, we should handle these cases.

http://llvm-reviews.chandlerc.com/D2061