Fix the size of the _Unwind_Exception struct

On OSX 32-bit, the private fields are 5 words long, not 2. I found this
segfaulting before this change, and after this change it no longer segfaulted.
auto merge of #11947 : alexcrichton/rust/osx-unwind, r=brson

On OSX 32-bit, the private fields are 5 words long, not 2. I found this
segfaulting before this change, and after this change it no longer segfaulted.