[scripts][python] Move a few more scripts from python2 to python3

Turns out they were already python3 safe, so just switch to requested
version over.

arch/arm/stackusage

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 import sys
 import re

platform/zynq/mkbootheader.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 # generates Zynq bootrom header from input payout