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

[Benoît Amiaux via llvm-commits]

bamiaux wrote:

A few thoughts:
- linux archives add flang & mir to built projects. I didn't change this.
- lld.lib is not distributed in this tarball, but I need plugin support, as done in this project https://github.com/jamesmth/llvm-project. Maybe I should open a new issue ?
- I didn't add any x86 or arm64 tarball. It's easy to do but I thought I should get some feedback & maybe finalize x64 first ?

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