systemc: Generalize gem5 style event scheduling.

systemc: Generalize gem5 style event scheduling.

These events are either scheduled directly, or if no event queue is
yet available they're recorded in a map to schedule later. Since this
was used in a few places (and should have been used for the ready
event), this change moves it into some common functions which remove
some duplication and abstract away this detail.

Change-Id: I4320d7296f4f72344539b2b4b2564a6a27576dd8
Reviewed-on: https://gem5-review.googlesource.com/12219
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>