ext: Updated Pybind11 to version 2.4.1.

This updates Pybind11 from version 2.2.1 to version 2.4.1. This fixes
warning/error received when "<experiment/optional>" is used when
compiling using c++14 with clang. It should be noted that
"ext/pybind11/include/pybind11/std.h" has been changed to include a fix
added by commit ba42457254. This is
necessary to avoid build errors.

Built: Linux (gcc, c++11) and MacOS (clang, c++14).
Tested: Ran quick tests for X86, ARM, and RISC-V.
Deprecates: https://gem5-review.googlesource.com/c/public/gem5/+/21019
Change-Id: Ie9783511cb6be50136076a55330e645f4f36d075
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/21119
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>

ext/pybind11/tests/test_async.cpp

@@ -0,0 +1,26 @@
+/*
+    tests/test_async.cpp -- __await__ support
+
+    Copyright (c) 2019 Google Inc.
+
+    All rights reserved. Use of this source code is governed by a
+    BSD-style license that can be found in the LICENSE file.
+*/
+
+#include "pybind11_tests.h"
+
+TEST_SUBMODULE(async_module, m) {
+    struct DoesNotSupportAsync {};
+    py::class_<DoesNotSupportAsync>(m, "DoesNotSupportAsync")
+        .def(py::init<>());
+    struct SupportsAsync {};
+    py::class_<SupportsAsync>(m, "SupportsAsync")
+        .def(py::init<>())
+        .def("__await__", [](const SupportsAsync& self) -> py::object {
+            static_cast<void>(self);
+            py::object loop = py::module::import("asyncio.events").attr("get_event_loop")();
+            py::object f = loop.attr("create_future")();
+            f.attr("set_result")(5);
+            return f.attr("__await__")();
+        });
+}