libc: Always declare getenv, link/symlink and atexit/on_exit

since many c++ library implementation reference these symbols by using ::xxx but never
really use them, the declaration avoid the unused code is pulled into the final binary

Signed-off-by: Xiang Xiao <xiaoxiang@xiaomi.com>
Change-Id: Idd7bf9a1e09b77a6b21f900cd3ede08a1cc82d86

include/cxx/cstdlib

@@ -40,7 +40,6 @@ namespace std
   using ::rand;
   using ::random;
 
-#ifndef CONFIG_DISABLE_ENVIRON
   // Environment variable support
 
   using ::get_environ_ptr;
@@ -49,18 +48,13 @@ namespace std
   using ::clearenv;
   using ::setenv;
   using ::unsetenv;
-#endif
 
   // Process exit functions
 
   using ::exit;
   using ::abort;
-#ifdef CONFIG_SCHED_ATEXIT
   using ::atexit;
-#endif
-#ifdef CONFIG_SCHED_ONEXIT
   using ::on_exit;
-#endif
 
 #ifndef __KERNEL__
   // System command