[llvm] [cmake] Fix ninja clean failure for cross-compilation native build dir (PR #200641)

[Martin Storsjö via llvm-commits]

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

This looks reasonable to me, thanks! Avoiding the spurious errors on `ninja clean` would indeed be nice.

I'll CC @petrhosek on this still, in case he wants to comment on it - otherwise we can consider merging it in a few days.

https://github.com/llvm/llvm-project/pull/200641