sim: switch config.h usage to defs.h

The defs.h header will take care of including the various config.h
headers.  For now, it's just config.h, but we'll add more when we
integrate gnulib in.

This header should be used instead of config.h, and should be the
first include in every .c file.  We won't rely on the old behavior
where we expected files to include the port's sim-main.h which then
includes the common sim-basics.h which then includes config.h.  We
have a ton of code that includes things before sim-main.h, and it
sometimes needs to be that way.  Creating a dedicated header avoids
the ordering mess and implicit inclusion that shows up otherwise.

sim/example-synacor/ChangeLog

@@ -1,3 +1,7 @@
+2021-05-16  Mike Frysinger  <vapier@gentoo.org>
+
+	* interp.c, sim-main.c: Replace config.h include with defs.h.
+
 2021-05-16  Mike Frysinger  <vapier@gentoo.org>
 
 	* config.in, configure: Regenerate.

sim/example-synacor/interp.c

@@ -25,7 +25,8 @@
    For more specific details on these functions, see the sim/sim.h header
    file.  */
 
-#include "config.h"
+/* This must come before any other includes.  */
+#include "defs.h"
 
 #include "sim-main.h"
 #include "sim-options.h"

sim/example-synacor/sim-main.c

@@ -21,7 +21,8 @@
 /* This file contains the main simulator decoding logic.  i.e. everything that
    is architecture specific.  */
 
-#include "config.h"
+/* This must come before any other includes.  */
+#include "defs.h"
 
 #include "sim-main.h"