sim: add some stdlib.h includes

When trying to compile GDB with --target=avr, with gcc 9.2.0, I am
getting a bunch of:

    /home/simark/src/binutils-gdb/sim/avr/../common/nrun.c:94:7: error: implicit declaration of function ‘abort’ [-Werror=implicit-function-declaration]
       94 |       abort ();
          |       ^~~~~
    /home/simark/src/binutils-gdb/sim/avr/../common/nrun.c:94:7: error: incompatible implicit declaration of built-in function ‘abort’ [-Werror]
    /home/simark/src/binutils-gdb/sim/avr/../common/nrun.c:94:7: note: include ‘<stdlib.h>’ or provide a declaration of ‘abort’

I did what the compiler told me and added the relevant includes in the
problematic files.

sim/common/ChangeLog:

	* nrun.c: Include stdlib.h.
	* sim-core.c: Likewise.
	* sim-engine.c: Likewise.
	* sim-io.c: Likewise.
	* sim-module.c: Likewise.
	* sim-reason.c: Likewise.

sim/common/ChangeLog

@@ -1,3 +1,12 @@
+2020-01-19  Simon Marchi  <simon.marchi@polymtl.ca>
+
+	* nrun.c: Include stdlib.h.
+	* sim-core.c: Likewise.
+	* sim-engine.c: Likewise.
+	* sim-io.c: Likewise.
+	* sim-module.c: Likewise.
+	* sim-reason.c: Likewise.
+
 2019-12-19  Tom Tromey  <tromey@adacore.com>
 
 	PR build/24572:

sim/common/nrun.c

@@ -20,6 +20,7 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
 #endif
 
 #include <signal.h>
+#include <stdlib.h>
 
 /* For strsignal.  */
 #ifdef HAVE_STRING_H

sim/common/sim-core.c

@@ -30,6 +30,8 @@
 #include "sim-hw.h"
 #endif
 
+#include <stdlib.h>
+
 /* "core" module install handler.
 
    This is called via sim_module_install to install the "core"

sim/common/sim-engine.c

@@ -17,11 +17,12 @@ GNU General Public License for more details.
 You should have received a copy of the GNU General Public License
 along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
 
-#include <stdio.h>
-
 #include "sim-main.h"
 #include "sim-assert.h"
 
+#include <stdio.h>
+#include <stdlib.h>
+
 /* Get the run state.
    REASON/SIGRC are the values returned by sim_stop_reason.
    ??? Should each cpu have its own copy?  */

sim/common/sim-io.c

@@ -33,6 +33,8 @@
 #include <unistd.h>
 #endif
 
+#include <stdlib.h>
+
 /* Define the rate at which the simulator should poll the host
    for a quit. */
 #ifndef POLL_QUIT_INTERVAL

sim/common/sim-module.c

@@ -36,6 +36,8 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
 
 #include "libiberty.h"
 
+#include <stdlib.h>
+
 /* List of all modules.  */
 static MODULE_INSTALL_FN * const modules[] = {
   standard_install,

sim/common/sim-reason.c

@@ -20,6 +20,8 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
 #include "sim-main.h"
 #include "sim-assert.h"
 
+#include <stdlib.h>
+
 /* Generic implementation of sim_stop_reason */
 
 void