scons,python: Split the marshal binary into a c++ wrapper and script.

The new c++ wrapper is called gem5py, and will run any python script
using gem5's embedded python interpreter. The "marshal" functionality is
split out into a separate python script gem5py can run.

The command line for gem5py should look like this:

gem5py ${SCRIPT TO RUN} ${ARGS TO THE SCRIPT}

So, for instance, to marshal a file called foo.py, the command line
might look like this:

gem5py python/marshal.py foo.py

Also, this change reorders the sources for the python embedding action
and limits the max_sources for Transform() to 1, so that it just shows
the python file being embedded and not gem5py or the marshal.py script.
Those are still sources so dependency tracking works correctly, but they
are also implied and just add visual noise to the build output.

Change-Id: I7ae6bd114973ae44c3b634884b6dafc6577e0788
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/49392
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>

src/python/gem5py.cc

@@ -0,0 +1,82 @@
+/*
+ * 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.
+ */
+
+#include <pybind11/embed.h>
+
+#include <cstdlib>
+#include <iostream>
+
+namespace py = pybind11;
+
+/*
+ * This wrapper program runs python scripts using the python interpretter which
+ * will be built into gem5. Its first argument is the script to run, and then
+ * all subsequent arguments are passed to the python script as its argv.
+ */
+
+int
+main(int argc, const char **argv)
+{
+    py::scoped_interpreter guard;
+
+    // Embedded python doesn't set up sys.argv, so we'll do that ourselves.
+    py::list py_argv;
+    auto sys = py::module::import("sys");
+    if (py::hasattr(sys, "argv")) {
+        // sys.argv already exists, so grab that.
+        py_argv = sys.attr("argv");
+    } else {
+        // sys.argv doesn't exist, so create it.
+        sys.add_object("argv", py_argv);
+    }
+    // Clear out argv just in case it has something in it.
+    py_argv.attr("clear")();
+
+    if (argc < 2) {
+        std::cerr << "Usage: gem5py SCRIPT [arg] ..." << std::endl;
+        std::exit(1);
+    }
+
+    // Fill it with our argvs.
+    for (int i = 1; i < argc; i++)
+        py_argv.append(argv[i]);
+
+    // Actually call the script.
+    py::eval_file(argv[1]);
+
+    return 0;
+}