sim: invert sim_state storage

Currently all ports have to declare sim_state themselves in their
sim-main.h and then embed the common sim_state_base & sim_cpu in it.
This dynamic makes it impossible to share common object code among
multiple ports because the core data structure is always different.

Let's invert this relationship: common code declares sim_state, and
if the port actually needs state on a per-instance basis, it can use
the new arch_data field for it.  Most ports don't actually use it,
so they don't need to declare anything at all.

This is the first in a series of changes: it adds a define to select
between the old & new layouts, then converts all the ports that don't
need custom state over to the new layout.

sim/common/sim-utils.c

@@ -48,8 +48,8 @@ zalloc (unsigned long size)
 /* Allocate a sim_state struct.  */
 
 SIM_DESC
-sim_state_alloc (SIM_OPEN_KIND kind,
-		 host_callback *callback)
+sim_state_alloc_extra (SIM_OPEN_KIND kind, host_callback *callback,
+		       size_t extra_bytes)
 {
   SIM_DESC sd = ZALLOC (struct sim_state);
 
@@ -57,6 +57,11 @@ sim_state_alloc (SIM_OPEN_KIND kind,
   STATE_CALLBACK (sd) = callback;
   STATE_OPEN_KIND (sd) = kind;
 
+#ifdef SIM_HAVE_COMMON_SIM_STATE
+  if (extra_bytes)
+    STATE_ARCH_DATA (sd) = zalloc (extra_bytes);
+#endif
+
 #if 0
   {
     int cpu_nr;