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

[via llvm-commits]

zmodem wrote:

> 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) ?

I've added it to the 18.1.0-rc3 release (see also https://discourse.llvm.org/t/llvm-18-1-0-rc3-tagged/77116/4) and will upload the new tarball for future release builds also.

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