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>

build_tools/marshal.py

@@ -0,0 +1,64 @@
+# Copyright (c) 2019 ARM Limited
+# All rights reserved
+#
+# The license below extends only to copyright in the software and shall
+# not be construed as granting a license to any other intellectual
+# property including but not limited to intellectual property relating
+# to a hardware implementation of the functionality of the software
+# licensed hereunder.  You may use the software subject to the license
+# terms below provided that you ensure that this notice is replicated
+# unmodified and in its entirety in all distributions of the software,
+# modified or unmodified, in source code or in binary form.
+#
+# Redistribution and use in source and binary forms, with or without
+# modification, are permitted provided that the following conditions are
+# met: redistributions of source code must retain the above copyright
+# notice, this list of conditions and the following disclaimer;
+# redistributions in binary form must reproduce the above copyright
+# notice, this list of conditions and the following disclaimer in the
+# documentation and/or other materials provided with the distribution;
+# neither the name of the copyright holders nor the names of its
+# contributors may be used to endorse or promote products derived from
+# this software without specific prior written permission.
+#
+# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+# "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+# LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+# A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+# OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+# SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+# LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+# DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+# THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+# (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+# OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+"""Marshal another python script.
+
+This script compiles another script, marshals the resulting code object, and
+writes it to stdout. Marshalling an object in this sense is essentially what
+would happen to write it out in a pyc file.
+
+This code object can then be read back in by another python script later and
+executed, without having to have access to the original file. This is how
+python code is embedded into the gem5 binary.
+
+The output of the marshal module is *not* generally compatible accross python
+interpretters, and so the exact same interpretter should be used both to run
+this script, and to read in and execute the marshalled code later.
+"""
+
+import marshal
+import sys
+
+if len(sys.argv) < 2:
+    print(f"Usage: {sys.argv[0]} PYSOURCE", file=sys.stderr)
+    sys.exit(1)
+
+source = sys.argv[1]
+with open(source, 'r') as f:
+    src = f.read()
+
+compiled = compile(src, source, 'exec')
+marshalled = marshal.dumps(compiled)
+sys.stdout.buffer.write(marshalled)