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

[Michael Kruse via llvm-commits]

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

OK to commit as-is. It's low impact. `LLVM_TABLEGEN_FLAGS` can be considered unrelated.

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