libunwind: Properly align _Unwind_Exception.

_Unwind_Exception is required to be double word aligned.  GCC has
interpreted this to mean "use the maximum useful alignment for the
target" so follow that lead.

Obtained from:	LLVM review D22543

contrib/llvm/projects/libunwind/include/unwind.h

@@ -128,7 +128,7 @@ struct _Unwind_Exception {
   // added for binary compatibility.
   uint32_t reserved[3];
 #endif
-};
+} __attribute__((__aligned__));
 
 typedef _Unwind_Reason_Code (*_Unwind_Stop_Fn)
     (int version,