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

[Dirk Pranke via llvm-commits]

dpranke wrote:

> > Do you want to update the comment right above too?
> 
> (Looks like this wasn't done yet.)

Whoops. Fixed.

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