sim: move default model to the runtime sim state

This kills off another compile-time option by moving the setting to
the individual arch runtimes.  This will allow dynamic selection by
the arch when doing a single build with multiple arches.

The sim_model_init rework is a little funky.  In the past it was
disabled entirely if no default model was set.  We maintain the
spirit of the logic by gating the fallback logic on whether the
port has defined any models.

sim/configure

@@ -640,7 +640,6 @@ am__EXEEXT_TRUE
 LTLIBOBJS
 SIM_RX_CYCLE_ACCURATE_FLAGS
 sim_reserved_bits
-sim_default_model
 sim_scache
 sim_float
 sim_bitsize
@@ -12180,7 +12179,7 @@ else
   lt_dlunknown=0; lt_dlno_uscore=1; lt_dlneed_uscore=2
   lt_status=$lt_dlunknown
   cat > conftest.$ac_ext <<_LT_EOF
-#line 12183 "configure"
+#line 12182 "configure"
 #include "confdefs.h"
 
 #if HAVE_DLFCN_H
@@ -12286,7 +12285,7 @@ else
   lt_dlunknown=0; lt_dlno_uscore=1; lt_dlneed_uscore=2
   lt_status=$lt_dlunknown
   cat > conftest.$ac_ext <<_LT_EOF
-#line 12289 "configure"
+#line 12288 "configure"
 #include "confdefs.h"
 
 #if HAVE_DLFCN_H
@@ -14674,7 +14673,6 @@ fi
 
 
 
-
 { $as_echo "$as_me:${as_lineno-$LINENO}: checking whether sim rx should be cycle accurate" >&5
 $as_echo_n "checking whether sim rx should be cycle accurate... " >&6; }
 # Check whether --enable-sim-rx-cycle-accurate was given.