python: Mimic Python 3's -P flag in gem5

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

src/python/m5/main.py

@@ -193,6 +193,14 @@ def parse_options():
         callback=collect_args,
     )
 
+    option(
+        "-P",
+        action="store_true",
+        default=False,
+        help="Don't prepend the script directory to the system path. "
+        "Mimics Python 3's `-P` option.",
+    )
+
     option(
         "-s",
         action="store_true",
@@ -601,7 +609,11 @@ def main():
         sys.argv = ["-c"] + options.c[1]
         scope = {"__name__": "__m5_main__"}
     else:
-        sys.path = [os.path.dirname(sys.argv[0])] + sys.path
+        # If `-P` was used (`options.P == true`), don't prepend the script
+        # directory to the `sys.path`. This mimics Python 3's `-P` option
+        # (https://docs.python.org/3/using/cmdline.html#cmdoption-P).
+        if not options.P:
+            sys.path = [os.path.dirname(sys.argv[0])] + sys.path
         filename = sys.argv[0]
         filedata = open(filename, "r").read()
         filecode = compile(filedata, filename, "exec")