x86: Expose thread features in /proc/$PID/status

Applications and loaders can have logic to decide whether to enable
shadow stack. They usually don't report whether shadow stack has been
enabled or not, so there is no way to verify whether an application
actually is protected by shadow stack.

Add two lines in /proc/$PID/status to report enabled and locked features.

Since, this involves referring to arch specific defines in asm/prctl.h,
implement an arch breakout to emit the feature lines.

[Switched to CET, added to commit log]

Co-developed-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Mike Rapoport (IBM) <rppt@kernel.org>
Tested-by: Pengfei Xu <pengfei.xu@intel.com>
Tested-by: John Allen <john.allen@amd.com>
Tested-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/all/20230613001108.3040476-37-rick.p.edgecombe%40intel.com

include/linux/proc_fs.h

@@ -159,6 +159,7 @@ int proc_pid_arch_status(struct seq_file *m, struct pid_namespace *ns,
 #endif /* CONFIG_PROC_PID_ARCH_STATUS */
 
 void arch_report_meminfo(struct seq_file *m);
+void arch_proc_pid_thread_features(struct seq_file *m, struct task_struct *task);
 
 #else /* CONFIG_PROC_FS */