build: Request python2 for all directly executed python binaries

Some systems point python to python3 instead of python2 - explicitly
request python2 to avoid conflicts.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

scripts/checkstack.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python2
 # Script that tries to find how much stack space each function in an
 # object is using.
 #