systemc: Add a distinct async_request_update mechanism.

This mechanism had just been plumbed into the regular request_update,
but that doesn't have any thread safety which is the whole point of
async_request_update. This new mechanism puts async update requests
into their own list which is checked any time normal updates happen.

The delta cycle which triggers those updates must happen through some
other means which will usually be ok. The exact timing of the update
is undefined, so it would be legal for it to either not be recognized
before the impending end of the simulation, or for it to get picked up
by subsequent activity. If there isn't subsequent activity but the
simulation also doesn't end, for instance if there are only gem5 events
left, then that update could be lost. That is an unresolved issue.

It would be nice to schedule a "ready" event if async updates were
added which would ensure they wouldn't starve. Unfortunately that
requires the event queue lock, and in practice it's been found that a
systemc process might block, effectively holding the event queue lock,
while it waits for some asyncrhonous update to give it something to do.
This effectively deadlocks the system since the update is blocked on
the lock the main thread holds, and the main thread is blocked waiting
for the update.

Change-Id: I580303db01673faafc2e63545b6a69b3327a521c
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/18288
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>

systemc: Improve scheduler cleanup.

Make the scheduler clear itself out when it's destructed to ensure that
nobody will try to use it after it's gone away. Also make sure there
are no pending events which might refer to it as well, either systemc
events or gem5 events.

Change-Id: I12dadc06bd9db7016a8dc0c1827b3e630b0d23d5
Reviewed-on: https://gem5-review.googlesource.com/12222
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

systemc: Track exports and prim channels, and call their callbacks.

Also call the callbacks on the ports which were already being tracked.

Change-Id: I5ba8ea366e87fc48b58712f35b93c27bccf92cb3
Reviewed-on: https://gem5-review.googlesource.com/12210
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

systemc: Implement channel updates and rework the scheduler.

This change implements channel updates, and also reworks the scheduler
to delegate more to the gem5 event queue by taking advantage of
event priorities to ensure things happen in the right order. There's
a lengthy comment in scheduler.hh describes how that all works.

Change-Id: I5dee71b86b2e612bb720a4429f3a72e4b7c6d01f
Reviewed-on: https://gem5-review.googlesource.com/11710
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>