PATCH: Cast to _Unwind_Exception_Class when setting exc.exception_class

[Roland McGrath]

Andreas correctly points out that it really should have parens, as should
the x86_64 THREAD_SETMEM (which doesn't).  With that change, it's fine.

But I'll note that it really seems like a compiler bug to me that an "A"
constraint would ever fail to set %edx.  (Not that I expect it to be fixed.)


Thanks,
Roland