Driver: add target definition for Windows on ARM

This introduces the definitions needed for the Windows on ARM target.  Add
target definitions for both the MSVC environment and the MSVC + Itanium C++ ABI
environment.  The Visual Studio definitions correspond to the definitions
provided by Visual Studio 2012.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@205650 91177308-0d34-0410-b5e6-96231b3b80d8