[libc] have fputs return the number of bytes written

Not strictly according to spec but it matches the way the unit test was
written.

lib/libc/stdio.c

@@ -247,7 +247,7 @@ int fputs(const char *s, FILE *fp) {
     if (written == 0) {
         return EOF;
     }
-    return 0;
+    return written;
 }
 
 int fgetc(FILE *fp) {