elf: Introduce __tls_init_tp for second-phase TCB initialization

TLS_INIT_TP is processor-specific, so it is not a good place to
put thread library initialization code (it would have to be repeated
for all CPUs).  Introduce __tls_init_tp as a separate function,
to be called immediately after TLS_INIT_TP.  Move the existing
stack list setup code for NPTL to this function.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

elf/dl-tls_init_tp.c

@@ -0,0 +1,24 @@
+/* Completion of TCB initialization after TLS_INIT_TP.  Generic version.
+   Copyright (C) 2021 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <ldsodefs.h>
+
+void
+__tls_init_tp (void)
+{
+}