[llvm] [gn] Use exec_script_allowlist in //llvm/utils/gn/.gn (PR #134172)

[Nico Weber via llvm-commits]

https://github.com/nico approved this pull request.

Thanks, looks great! I'll land this some time today, after making sure my bots have a new-enough GN.

https://github.com/llvm/llvm-project/pull/134172