libc: Implement realpath

specified here:
https://pubs.opengroup.org/onlinepubs/9699919799/functions/realpath.html

Signed-off-by: Xiang Xiao <xiaoxiang@xiaomi.com>
Change-Id: Ie52dcd1c5c5faa4b033901eedd7182bbb9473f7a

include/stdlib.h

@@ -167,6 +167,8 @@ static inline void _Exit(int s)
 int       system(FAR const char *cmd);
 #endif
 
+FAR char *realpath(FAR const char *path, FAR char *resolved);
+
 /* String to binary conversions */
 
 long      strtol(FAR const char *nptr, FAR char **endptr, int base);