Enable CFI on OS X.

Currently the output should be almost identical to the one produced by CodeGen
to make the transition easier.

The only two differences I know of are:

* Some files get an extra advance loc of size 0. This will be fixed when
relaxations are enabled.
* The optimization of declaring an EH symbol as an external variable is not
implemented. This is a subset of adding the nounwind attribute, so we if really
this at -O0 we should probably do it at the IL level.

llvm-svn: 130623