systemc: Add a "changeStamp" value to the scheduler.

This value is incremented after each delta cycle's evaluate stage and
after timed notifications happen. Its value is used by some channels
to determine whether certain events happened within the previous update
phase to implement the "event()", "posedge()", and "negedge()"
functions.

Change-Id: I9a73f0b5007dcbb6a74da9d666f28da1930b9d3d
Reviewed-on: https://gem5-review.googlesource.com/c/12452
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

src/systemc/ext/core/sc_prim.hh

@@ -38,6 +38,8 @@ namespace sc_gem5
 
 class Channel;
 
+uint64_t getChangeStamp();
+
 } // namespace sc_gem5
 
 namespace sc_core