systemc: Refactor reporting to prep for int based messages.

There's a deprecated reporting mechanism based on integer message ids,
and the reporting mechanism needs to be refactored a bit to make it
easier to support.

Some bookkeeping data structures were moved out to somewhere they
can be accessed by other code, obviating the non-standard get_handler
function.

Change-Id: Id427cd79be9ef0f3275fbac39ff047ab672fb3e0
Reviewed-on: https://gem5-review.googlesource.com/c/13318
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

src/systemc/ext/utils/sc_report_handler.hh

@@ -105,10 +105,6 @@ class sc_report_handler
     static void set_handler(sc_report_handler_proc);
     static void default_handler(const sc_report &, const sc_actions &);
     static sc_actions get_new_action_id();
-    // Nonstandard
-    // Returns the current handler so it can be restored if it needs to be
-    // changed temporarily.
-    static sc_report_handler_proc get_handler();
 
     static sc_report *get_cached_report();
     static void clear_cached_report();