Make the new winrt and dx abstractions multilib safe by making the variable names different between the 32 and 64 cases.  Add it to extra_dist so that it gets distributed, and clean up extra_dist to match

git-svn-id: svn+ssh://svn.code.sf.net/p/mingw-w64/code/trunk@6266 4407c894-4637-0410-b4f5-ada5f102cad1