aarch64: Mark swapcontext with indirect_return

Reviewed-by: Wilco Dijkstra <Wilco.Dijkstra@arm.com>
Reviewed-by: Carlos O'Donell <carlos@redhat.com>

sysdeps/aarch64/bits/indirect-return.h

@@ -0,0 +1,36 @@
+/* Definition of __INDIRECT_RETURN.  AArch64 version.
+   Copyright (C) 2024-2025 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 _UCONTEXT_H
+# error "Never include <bits/indirect-return.h> directly; use <ucontext.h> instead."
+#endif
+
+/* __INDIRECT_RETURN indicates that swapcontext may return via
+   an indirect branch.  This happens when GCS is enabled, so
+   add the attribute if available, otherwise returns_twice has
+   a similar effect, but it prevents some code transformations
+   that can cause build failures in some rare cases so it is
+   only used when GCS is enabled.  */
+#if __glibc_has_attribute (__indirect_return__)
+# define __INDIRECT_RETURN __attribute__ ((__indirect_return__))
+#elif __glibc_has_attribute (__returns_twice__) \
+      && defined __ARM_FEATURE_GCS_DEFAULT
+# define __INDIRECT_RETURN __attribute__ ((__returns_twice__))
+#else
+# define __INDIRECT_RETURN
+#endif