"S" is a keyword in 65816 mode only.

git-svn-id: svn://svn.cc65.org/cc65/trunk@3958 b7a2c559-68d2-44c3-8de9-860c34a00d81

src/ca65/scanner.c

@@ -1013,8 +1013,11 @@ Again:
 		    break;
 
 	        case 'S':
+                    if (CPU == CPU_65816) {
                         Tok = TOK_S;
                         return;
+                    }
+                    break;
 
     	     	case 'X':
      	     	    Tok = TOK_X;