Use usr/bin/env for all python bangpaths to enable virtualenv use.

Very useful with python3 as the default platform.

libsel4/tools/syscall_stub_gen.py

-#!/usr/bin/python
+#!/usr/bin/env python
 #
 # Copyright 2014, NICTA
 #

manual/tools/gen_env.py

-#!/usr/bin/python
+#!/usr/bin/env python
 #
 # Copyright 2014, NICTA
 #

tools/invocation_header_gen.py

-#!/usr/bin/python 
+#!/usr/bin/env python
 #
 # Copyright 2014, NICTA
 #

tools/syscall_header_gen.py

-#!/usr/bin/python 
+#!/usr/bin/env python
 #
 # Copyright 2014, NICTA
 #