Update the Windows packaging script.

As discussed on:
  https://discourse.llvm.org/t/build-llvm-release-bat-script-options/63146/6

Latest 7-zip versions (21.x) require administrator permissions
to create symbolic links.

Check if any of the following conditions is true:
- Version of 7-zip is 20.x or older
- Script is running with administrator permissions

Reviewed By: hans, thieta

Differential Revision: https://reviews.llvm.org/D129263