sim: Fix compile errors

The following change broke simulator testsuite with host GCC 13:
  commit 435ad222b3
  sim: warnings: compile build tools with -Werror too

Host GCC13 complains about missing function prototypes:

binutils/sim/testsuite/common/bits-gen.c:26:1: error: no previous prototype for ‘gen_struct’ [-Werror=missing-prototypes]
   26 | gen_struct (void)
      | ^~~~~~~~~~

Fix by making the functions static, which instructs the compiler that
there is no need for a prototype.

Signed-off-by: Dimitar Dimitrov <dimitar@dinux.eu>

sim/testsuite/common/alu-n-tst.h

@@ -27,7 +27,7 @@
 #define do_op_N      XCONCAT2(do_op_,N)
 
 
-void
+static void
 do_op_N (const alu_test *tst)
 {
   const alu_op *op;
@@ -120,7 +120,7 @@ do_op_N (const alu_test *tst)
 }
 
 
-const alu_test alu_N_tests[] = {
+static const alu_test alu_N_tests[] = {
 
   /* 0 + 0; 0 + 1; 1 + 0; 1 + 1 */
   { 0, { { "ADDC", 0 }, }, 0, 0, 0, },

sim/testsuite/common/bits-gen.c

@@ -22,7 +22,7 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
 #include <string.h>
 #include <unistd.h>
 
-void
+static void
 gen_struct (void)
 {
   printf ("\n");
@@ -44,7 +44,7 @@ gen_struct (void)
 }
 
 
-void
+static void
 gen_bit (int bitsize,
 	 int msb,
 	 const char *macro,
@@ -79,7 +79,7 @@ gen_bit (int bitsize,
 }
 
 
-void
+static void
 gen_enum (const char *macro,
 	  int nr_bits)
 {
@@ -96,7 +96,7 @@ gen_enum (const char *macro,
 }
 
 
-void
+static void
 gen_mask (int bitsize,
 	  const char *msb,
 	  const char *macro,
@@ -160,7 +160,7 @@ gen_mask (int bitsize,
 }
 
 
-void
+static void
 usage (int reason)
 {
   fprintf (stderr, "Usage:\n");

sim/testsuite/common/bits-tst.c

@@ -3,7 +3,7 @@
 /* Drive the bit test routines */
 
 
-long long
+static long long
 calc (const char *call,
       long long val,
       int row,
@@ -161,7 +161,7 @@ calc (const char *call,
 }
 
 
-int
+static int
 check_sext (int nr_bits,
 	    int msb_nr,
 	    const char *sexted,
@@ -214,7 +214,7 @@ check_sext (int nr_bits,
 }
 
 
-int
+static int
 check_rot (int nr_bits,
 	   const char *roted,
 	   const char *masked)
@@ -248,7 +248,7 @@ check_rot (int nr_bits,
 }
 
 
-int
+static int
 check_extract (int nr_bits,
 	       const char *extracted,
 	       const char *inserted,
@@ -277,7 +277,7 @@ check_extract (int nr_bits,
 }
 
 
-int
+static int
 check_bits (int call,
 	    test_spec **tests)
 {