Revert "tls: Move pthread key destructor to libc"

This reverts commit cc514d7791.

* It introduced a regression.
  https://github.com/apache/incubator-nuttx/issues/3868

* It seems conceptually wrong to have per-process data in
  the main thread's stack.

include/sys/syscall_lookup.h

@@ -294,6 +294,14 @@ SYSCALL_LOOKUP(telldir,                    1)
   SYSCALL_LOOKUP(shmdt,                    1)
 #endif
 
+#if CONFIG_TLS_NELEM > 0
+  SYSCALL_LOOKUP(tls_alloc,                0)
+  SYSCALL_LOOKUP(tls_free,                 1)
+  SYSCALL_LOOKUP(tls_get_set,              1)
+  SYSCALL_LOOKUP(tls_get_dtor,             1)
+  SYSCALL_LOOKUP(tls_set_dtor,             2)
+#endif
+
 /* The following are defined if pthreads are enabled */
 
 #ifndef CONFIG_DISABLE_PTHREAD