python: Recursively create checkpoint dir

While there was code present in "serialize.cc" to create the checkpoint
directory, it did not do recursively. This patch ensures all the
directories are created in a path to the checkpoint directory.

Change-Id: Ibcf7f800358fd89946f550b8cfb0cef8b51fceac

src/python/m5/simulate.py

@@ -318,6 +318,10 @@ def checkpoint(dir):
 
     drain()
     memWriteback(root)
+
+    # Recursively create the checkpoint directory if it does not exist.
+    os.makedirs(dir, exist_ok=True)
+
     print("Writing checkpoint")
     _m5.core.serializeAll(dir)