tools: update shebang in thumb-cc.py

Some distros, including Ubuntu/Debian, don't provide a "python"
command unless an additional package is installed. Using "python3"
explicitly should work everywhere.

Using "/usr/bin/env" respects the user's PATH, so they can choose
a specific Python version instead of forcing the systemwide copy.

Change-Id: I02e3ead92939c42df6def79dec3dc2a62c62b1c3

tools/thumb-cc.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python3
 # -*- coding: utf8 -*-
 #             __________               __   ___.
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