nptl: Add <thread_pointer.h> for LoongArch

This will be required by the rseq extensible ABI implementation on all
Linux architectures exposing the '__rseq_size' and '__rseq_offset'
symbols to set the initial value of the 'cpu_id' field which can be used
by applications to test if rseq is available and registered. As long as
the symbols are exposed it is valid for an application to perform this
test even if rseq is not yet implemented in libc for this architecture.

Both code paths are compile tested with build-many-glibcs.py but I don't
have access to any hardware to run the tests.

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Reviewed-by: Arjun Shankar <arjun@redhat.com>

sysdeps/loongarch/nptl/thread_pointer.h

@@ -0,0 +1,36 @@
+/* __thread_pointer definition.  loongarch version.
+   Copyright (C) 2021-2024 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/>.  */
+
+#ifndef _SYS_THREAD_POINTER_H
+#define _SYS_THREAD_POINTER_H
+
+#include <sys/cdefs.h>
+
+static inline void *
+__thread_pointer (void)
+{
+#if __glibc_has_builtin (__builtin_thread_pointer)
+  return __builtin_thread_pointer ();
+#else
+  void *__thread_register;
+  __asm__ ("move %0, $tp" : "=r" (__thread_register));
+  return __thread_register;
+#endif
+}
+
+#endif /* _SYS_THREAD_POINTER_H */