Introduce <pointer_guard.h>, extracted from <sysdep.h>

This allows us to define a generic no-op version of PTR_MANGLE and
PTR_DEMANGLE.  In the future, we can use PTR_MANGLE and PTR_DEMANGLE
unconditionally in C sources, avoiding an unintended loss of hardening
due to missing include files or unlucky header inclusion ordering.

In i386 and x86_64, we can avoid a <tls.h> dependency in the C
code by using the computed constant from <tcb-offsets.h>.  <sysdep.h>
no longer includes these definitions, so there is no cyclic dependency
anymore when computing the <tcb-offsets.h> constants.

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

sysdeps/unix/sysv/linux/powerpc/pointer_guard.h

@@ -0,0 +1,55 @@
+/* Pointer obfuscation implenentation.  PowerpC version.
+   Copyright (C) 2005-2022 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 POINTER_GUARD_H
+#define POINTER_GUARD_H
+
+#if IS_IN (rtld)
+/* We cannot use the thread descriptor because in ld.so we use setjmp
+   earlier than the descriptor is initialized.  */
+# include <sysdeps/generic/pointer_guard.h>
+#else
+# ifdef __ASSEMBLER__
+#  if defined(__PPC64__) || defined(__powerpc64__)
+#   define LOAD  ld
+#   define TPREG r13
+#  else
+#   define LOAD  lwz
+#   define TPREG r2
+#  endif
+#  define PTR_MANGLE(reg, tmpreg) \
+        LOAD    tmpreg,POINTER_GUARD(TPREG); \
+        xor     reg,tmpreg,reg
+#  define PTR_MANGLE2(reg, tmpreg) \
+        xor     reg,tmpreg,reg
+#  define PTR_MANGLE3(destreg, reg, tmpreg) \
+        LOAD    tmpreg,POINTER_GUARD(TPREG); \
+        xor     destreg,tmpreg,reg
+#  define PTR_DEMANGLE(reg, tmpreg) PTR_MANGLE (reg, tmpreg)
+#  define PTR_DEMANGLE2(reg, tmpreg) PTR_MANGLE2 (reg, tmpreg)
+#  define PTR_DEMANGLE3(destreg, reg, tmpreg) PTR_MANGLE3 (destreg, reg, tmpreg)
+# else
+#  include <stdint.h>
+#  include <tls.h>
+#  define PTR_MANGLE(var) \
+  (var) = (__typeof (var)) ((uintptr_t) (var) ^ THREAD_GET_POINTER_GUARD ())
+#  define PTR_DEMANGLE(var)     PTR_MANGLE (var)
+# endif
+#endif
+
+#endif /* POINTER_GUARD_H */