Headers: give _Unwind_Control_Block double-word alignment

The C++ ABI requires that the exception object (which under AEABI is the
`_Unwind_Control_Block`) is double-word aligned.  The attribute was
applied to the `_Unwind_Exception` type, but not the
`_Unwind_Control_Block`.  This should fix the libunwind test for the
alignment of the exception type.

llvm-svn: 311563

clang/lib/Headers/unwind.h

@@ -149,7 +149,7 @@ struct _Unwind_Control_Block {
     uint32_t reserved1;
   } pr_cache;
   long long int : 0; /* force alignment of next item to 8-byte boundary */
-};
+} __attribute__((__aligned__));
 #else
 struct _Unwind_Exception {
   _Unwind_Exception_Class exception_class;