scons,python: Move the marshal.py script into build_tools.

It's really a part of the build system and not part of gem5, and so it
should probably live outside of the main tree. It would be confusing to
have a bunch of python scripts which don't end up inside gem5 alongside
a bunch of ones that do in src/python.

The directory is called build_tools instead of build so it doesn't get
confused with an actual build output directory.

Change-Id: Ie12475a15517508dc2044f0ca4db71a601b7ab6d
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/49393
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>

src/SConscript

@@ -77,11 +77,13 @@ def GdbXml(xml_id, symbol):
 class Source(SourceFile):
     pass
 
+build_tools = Dir('#build_tools')
+
 # Build a small helper that marshals the Python code using the same version
 # of Python as gem5. This is in an unorthodox location to avoid building it
 # for every variant.
 gem5py = gem5py_env.Program('gem5py', 'python/gem5py.cc')[0]
-marshal_py = Dir('python').File('marshal.py')
+marshal_py = build_tools.File('marshal.py')
 
 # Embed python files.  All .py files that have been indicated by a
 # PySource() call in a SConscript need to be embedded into the M5