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>

util/cpt_upgrader.py

@@ -218,8 +218,12 @@ def process_file(path, **kwargs):
         change = True
 
         cpt.remove_option('root', 'cpt_ver')
+    # @todo The 'Globals' option is deprecated, and should be removed in the
+    # future
     elif cpt.has_option('Globals','version_tags'):
         tags = set((''.join(cpt.get('Globals','version_tags'))).split())
+    elif cpt.has_option('root.globals','version_tags'):
+        tags = set((''.join(cpt.get('root.globals','version_tags'))).split())
     else:
         print("fatal: no version information in checkpoint")
         exit(1)
@@ -253,7 +257,7 @@ def process_file(path, **kwargs):
         verboseprint("...nothing to do")
         return
 
-    cpt.set('Globals', 'version_tags', ' '.join(tags))
+    cpt.set('root.globals', 'version_tags', ' '.join(tags))
 
     # Write the old data back
     verboseprint("...completed")