[libcxxabi] Align unwindHeader on a double-word boundary.

r276215 made a change to annotate _Unwind_Exception with attribute
"aligned" so that an exception object following field __cxa_exception
is sufficiently aligned. This fix hasn't been incorporated to unwind.h
on Darwin since it is an ABI breaking change.

Instead of annotating struct _Unwind_Exception with the attribute, this
commit annotates field unwindHeader of __cxa_exception. This ensures the
exception object is sufficiently aligned without breaking the ABI.

rdar://problem/25364625

Differential Revision: https://reviews.llvm.org/D33030

llvm-svn: 302763