CVE-2018-3639 (x86)

Adjust arch_prctl_get/set_spec_ctrl() to operate on tasks other than
current.

This is needed both for /proc/$pid/status queries and for seccomp (since
thread-syncing can trigger seccomp in non-current threads).
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Tyler Hicks <tyhicks@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
[juergh: Context adjustments.]
Signed-off-by: Juerg Haefliger <juergh@canonical.com>