Summary:
Update the generated `run_cmake.py` script to use `os.execve()` rather than
`subprocess.call()`, so that it now propagates CMake's return code back to its
caller.

Reviewed By: chadaustin

Differential Revision: D17089206

fbshipit-source-id: e01f05f492ccb842d4967e59fd0bc9a3e59b8a42