S/390: PR83420: Improve hotpatch option parsing.

With the attached patch we get rid of the following build failure:

/home/andreas/build/../gcc/gcc/config/s390/s390.c: In function ‘void
s390_option_override()’:
/home/andreas/build/../gcc/gcc/config/s390/s390.c:15361:16: error: ‘char*
strncpy(char*, const char*, size_t)’ specified bound 256 equals destination
size [-Werror=stringop-truncation]
        strncpy (s, opt->arg, 256);
        ~~~~~~~~^~~~~~~~~~~~~~~~~~

gcc/ChangeLog:

2017-12-18  Andreas Krebbel  <krebbel@linux.vnet.ibm.com>

	PR target/83420
	* config/s390/s390.c (s390_option_override): Avoid strncpy.

From-SVN: r255777

gcc/ChangeLog

+2017-12-18  Andreas Krebbel  <krebbel@linux.vnet.ibm.com>
+
+	PR target/83420
+	* config/s390/s390.c (s390_option_override): Avoid strncpy.
+
 2017-12-18  Richard Biener  <rguenther@suse.de>
 
 	PR tree-optimization/81877

gcc/config/s390/s390.c

@@ -15357,16 +15357,11 @@ s390_option_override (void)
 	    {
 	      int val1;
 	      int val2;
-	      char s[256];
-	      char *t;
+	      char *s = strtok (ASTRDUP (opt->arg), ",");
+	      char *t = strtok (NULL, "\0");
 
-	      strncpy (s, opt->arg, 256);
-	      s[255] = 0;
-	      t = strchr (s, ',');
 	      if (t != NULL)
 		{
-		  *t = 0;
-		  t++;
 		  val1 = integral_argument (s);
 		  val2 = integral_argument (t);
 		}