Headers: improve ARM EHABI coverage of unwind.h

Ensure that we define the `_Unwind_Control_Block` structure used on ARM
EHABI targets.  This is needed for building libc++abi with the unwind.h
from the resource dir.  A minor fallout of this is that we needed to
create a typedef for _Unwind_Exception to work across ARM EHABI and
non-EHABI targets.  The structure definitions here are based originally
on the documentation from ARM under the "Exception Handling ABI for the
ARM® Architecture" Section 7.2.  They are then adjusted to more closely
reflect the definition in libunwind from LLVM.  Those changes are
compatible in layout but permit easier use in libc++abi and help
maintain compatibility between libunwind and the compiler provided
definition.

llvm-svn: 309226