Python 3's `-P` flag, when set, means `sys.path` is not prepended with potentially unsafe paths: https://docs.python.org/3/using/cmdline.html#cmdoption-P This patch allows gem5 to mimic this. This is necesssary when using `mypy.stubgen` as it expects the Python Interpreter to have the `-P` flag. Change-Id: I456c8001d3ee1e806190dc37142566d50d54cc90