sim: Clean up and simplify main().

Use pybind11 to simplify the python parts, update some inaccurate
comments, rename m5Main to gem5Main, remove code which supported python
versions less than 3.

Change-Id: I139af7d3d1052cfbce779a87e34d7ce997876a60
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/49414
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/python/importer.cc

@@ -29,10 +29,13 @@
 #include "pybind11/pybind11.h"
 #include "python/m5ImporterCode.hh"
 
+#include "sim/init.hh"
+
 namespace py = pybind11;
 
 PYBIND11_EMBEDDED_MODULE(importer, m)
 {
+    m.def("_init_all_embedded", gem5::EmbeddedPython::initAll);
     py::str importer_code(
             reinterpret_cast<const char *>(gem5::Blobs::m5ImporterCode),
             gem5::Blobs::m5ImporterCode_len);