libc: tag missing functions in system headers.

This matches recent changes in the NDK header.
We enclose missing functions in #if 0 .. #endif blocks
with a clear "MISSING" in comments in order to locate
them later.

Change-Id: I87b3a62e777897e75c9243360fb0a82bcc53d9fb
diff --git a/libc/include/stdlib.h b/libc/include/stdlib.h
index f889159..97d8e46 100644
--- a/libc/include/stdlib.h
+++ b/libc/include/stdlib.h
@@ -50,7 +50,6 @@
 extern __noreturn void exit(int);
 extern __noreturn void abort(void);
 extern int atexit(void (*)(void));
-extern int on_exit(void (*)(int, void *), void *);
 
 extern char *getenv(const char *);
 extern int putenv(const char *);
@@ -164,6 +163,7 @@
 
 extern lldiv_t   lldiv(long long, long long);
 
+#if 1 /* MISSING FROM BIONIC - ENABLED FOR STLPort and libstdc++-v3 */
 /* make STLPort happy */
 extern int      mblen(const char *, size_t);
 extern size_t   mbstowcs(wchar_t *, const char *, size_t);
@@ -172,8 +172,14 @@
 /* Likewise, make libstdc++-v3 happy.  */
 extern int	wctomb(char *, wchar_t);
 extern size_t	wcstombs(char *, const wchar_t *, size_t);
+#endif /* MISSING */
+
 #define MB_CUR_MAX 1
 
+#if 0 /* MISSING FROM BIONIC */
+extern int on_exit(void (*)(int, void *), void *);
+#endif /* MISSING */
+
 __END_DECLS
 
 #endif /* _STDLIB_H_ */