[llvm] [cmake] Pass PYTHON_EXECUTABLE to native builds (PR #163574)

[Ross Burton via llvm-commits]

rossburton wrote:

Updated. You've not misunderstood. My impression is this was triggered during the great py2/py3 migration disaster, but I can see a number of edge cases where it's a problem.

https://github.com/llvm/llvm-project/pull/163574