More Windows build fixes, some tidying in mojo_main

- Forced x64 (to avoid .asm build problems)
- Removed Sky as they're not interested in building/maintaining Windows at this point
- Misc Windows compile errors

R=jam@chromium.org

Committed: https://chromium.googlesource.com/external/mojo/+/c2d46e5a3882f80ffa3d8d50a39fa0b5a1271628

Review URL: https://codereview.chromium.org/700763002