Log Message:
lang/llvm: Fix Python dependency.

This ships a script llvm-lit with a Python interpreter baked into it
now, so Python is no longer tool-only.  Hack around the #! line for
cross-builds when PYTHONBIN != TOOL_PYTHONBIN.

If the intent is to ship llvm without a runtime Python dependency, we
either need to nix llvm-lit or split it into a separate package.