x86-64/cet: Make CET feature check specific to Linux/x86

CET feature bits in TCB, which are Linux specific, are used to check if
CET features are active.  Move CET feature check to Linux/x86 directory.
Reviewed-by: Noah Goldstein <goldstein.w.n@gmail.com>

sysdeps/unix/sysv/linux/x86/bits/platform/features.h

@@ -0,0 +1,48 @@
+/* Inline functions for x86 CPU features.
+   This file is part of the GNU C Library.
+   Copyright (C) 2024 Free Software Foundation, Inc.
+
+   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_PLATFORM_X86_H
+# error "Never include <bits/platform/features.h> directly; use <sys/platform/x86.h> instead."
+#endif
+
+/* Bits in the feature_1 field in TCB.  */
+
+enum
+{
+  x86_feature_1_ibt		= 1U << 0,
+  x86_feature_1_shstk		= 1U << 1
+};
+
+static __inline__ _Bool
+x86_cpu_cet_active (unsigned int __index)
+{
+#ifdef __x86_64__
+  unsigned int __feature_1;
+# ifdef __LP64__
+  __asm__ ("mov %%fs:72, %0" : "=r" (__feature_1));
+# else
+  __asm__ ("mov %%fs:40, %0" : "=r" (__feature_1));
+# endif
+  if (__index == x86_cpu_IBT)
+    return __feature_1 & x86_feature_1_ibt;
+  else
+    return __feature_1 & x86_feature_1_shstk;
+#else
+  return false;
+#endif
+}