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