Update build_llvm_package.bat

Update build_llvm_package.bat

I moved to a new machine and had to adjust a few things:

- Use %USERNAME% instead of %USER% (not sure why %USER% didn't work anymore)
- Update paths for using Python 3.6 instead of 3.5
- Skip building OpenMP which seems broken on Windows
- Work around new vsdevcmd.bat changing paths:
  https://developercommunity.visualstudio.com/content/problem/26780/vsdevcmdbat-changes-the-current-working-directory.html
- Build stage-0 compiler with MinSizeRel to work around VS 2017 bug:
  https://developercommunity.visualstudio.com/content/problem/139043/miscompile-in-trivial-c-program-with-155-preview-2.html

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@323427 91177308-0d34-0410-b5e6-96231b3b80d8