[llvm] [llvm] Fix cmake string expansion in CrossCompile.cmake (PR #138901)

[Petr Hosek via llvm-commits]

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

Thanks, we have other places in LLVM where we set these variables and ideally we would make the same change consistently everywhere.

https://github.com/llvm/llvm-project/pull/138901