LLDBStandalone was calling FindPythonInterp with 3.x enabled which
runs into an issue with all threads stuck waiting on Darwin and Linux.
The quick fix is to disable finding python3.x.
While restructuring Python, I figured I'd clean up the
FindPython{Interp,Libs} usage throughout the project.
See https://stackoverflow.com/questions/1242904/finding-python-site-packages-directory-with-cmake
If we can use CMake 3.12, use FindPython. If we can't, use execute_process.