[libc++] run.py: Create directory hierarchy if it does not exist

This is useful when replaying tests after the fact and the directories
don't exist anymore.
diff --git a/libcxx/utils/run.py b/libcxx/utils/run.py
index 8c3eaeb..7cdf652 100644
--- a/libcxx/utils/run.py
+++ b/libcxx/utils/run.py
@@ -45,7 +45,7 @@
 
     # Create the execution directory, and make sure we remove it at the end.
     try:
-        os.mkdir(args.execdir)
+        os.makedirs(args.execdir)
 
         # Ensure the file dependencies exist and copy them to the execution directory.
         for dep in args.dependencies: