sim,util: Remove event dependencies from serialize.hh

With this change serialize.hh is no longer responsible
for the (un)serialization of events. As a general rule,
rules to (un)serialize non-basic types should be defined
at the file that introduces that type. Therefore,
(UN)SERIALIZE_EVENT have been moved to eventq.hh.

Globals has a single instance which must be serialized
and unserialized. Instead of having a stray global
variable handled by Serialization, we pass its management
to Root. As a side effect, Globals is assigned its own
files: sim/globals.(cc/hh).

Finally, 'unserializeGlobals()' is removed, so that
Root can fully handle Globals' serialization. This
breaks checkpoint compatibility, so a checkpoint
upgrader is added.

Change-Id: I9c8e57306f83f9cc30ab2b745a4972755191bec4
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/43586
Tested-by: kokoro <noreply+kokoro@google.com>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>

src/python/pybind11/core.cc

@@ -308,7 +308,6 @@ pybind_init_core(py::module_ &m_native)
      */
     m_core
         .def("serializeAll", &Serializable::serializeAll)
-        .def("unserializeGlobals", &Serializable::unserializeGlobals)
         .def("getCheckpoint", [](const std::string &cpt_dir) {
             SimObject::setSimObjectResolver(&pybindSimObjectResolver);
             return new CheckpointIn(cpt_dir);