add #undef for some libc function

since it's useful to redirect these functions to others
sometime(e.g. validate the memory before write).

Change-Id: I6253a9231af8809e8362f4bc5a1bd67fb094c3b0

libs/libc/string/lib_strncasecmp.c

@@ -33,6 +33,7 @@
  ****************************************************************************/
 
 #ifndef CONFIG_ARCH_STRNCASECMP
+#undef strncasecmp /* See mm/README.txt */
 int strncasecmp(const char *cs, const char *ct, size_t nb)
 {
   int result = 0;