unwind: explicitly align `_Unwind_Control_Block`

The C++ ABI requires that the exception object is double-word aligned.
The alignment attribute was applied to the `_Unwind_Exception` type
which is used on non-EHABI targets.  On EHABI, the exception object type
is `_Unwind_Control_Block`.  Apply the explicit maximal alignment on the
type to ensure that the allocation has the correct alignment.

Resolves PR33858!

llvm-svn: 311562