[llvm] build_llvm_release.bat: add tarball export to x64 release (PR #79840)

[Benoît Amiaux via llvm-commits]

bamiaux wrote:

Thanks for the review !

Just in case, I've squashed & rebased it on latest upstream/main
I don't have commit access, so, yes, please merge it for me when you find the time.

Do you know whether this new archive can be added to the latest stable (17.0.6) or the next release (18.1.0-rc3 o rc4) ?

https://github.com/llvm/llvm-project/pull/79840