fix signed warnings

libc/string/generic/strchr.c

@@ -41,8 +41,8 @@ char *strchr (const char *s, int c_in)
 
   /* Handle the first few characters by reading one character at a time.
      Do this until CHAR_PTR is aligned on a longword boundary.  */
-  for (char_ptr = s; ((unsigned long int) char_ptr
-		      & (sizeof (longword) - 1)) != 0;
+  for (char_ptr = (const unsigned char *) s;
+       ((unsigned long int) char_ptr & (sizeof (longword) - 1)) != 0;
        ++char_ptr)
     if (*char_ptr == c)
       return (void *) char_ptr;

libc/string/generic/strchrnul.c

@@ -42,8 +42,8 @@ char *strchrnul (const char *s, int c_in)
 
   /* Handle the first few characters by reading one character at a time.
      Do this until CHAR_PTR is aligned on a longword boundary.  */
-  for (char_ptr = s; ((unsigned long int) char_ptr
-		      & (sizeof (longword) - 1)) != 0;
+  for (char_ptr = (const unsigned char *) s;
+       ((unsigned long int) char_ptr & (sizeof (longword) - 1)) != 0;
        ++char_ptr)
     if (*char_ptr == c || *char_ptr == '\0')
       return (void *) char_ptr;