libc: Implement umask function

Here is the spec:
https://pubs.opengroup.org/onlinepubs/009695399/functions/umask.html
Note: The simple implementaton is enough since
      NuttX doesn't really support the permission

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

include/sys/stat.h

@@ -168,6 +168,8 @@ int fstat(int fd, FAR struct stat *buf);
 int chmod(FAR const char *path, mode_t mode);
 int fchmod(int fd, mode_t mode);
 
+mode_t umask(mode_t mask);
+
 #undef EXTERN
 #if defined(__cplusplus)
 }