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/sim/SConscript

@@ -54,6 +54,7 @@ Source('py_interact.cc', add_tags='python')
 Source('eventq.cc')
 Source('futex_map.cc')
 Source('global_event.cc')
+Source('globals.cc')
 Source('init.cc', add_tags='python')
 Source('init_signals.cc')
 Source('main.cc', tags='main')