seccomp: Provide stub for __secure_computing()

To avoid #ifdeffery in the upcoming generic syscall entry work code provide
a stub for __secure_computing() as this is preferred over
secure_computing() because the TIF flag is already evaluated.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Kees Cook <keescook@chromium.org>
Link: https://lkml.kernel.org/r/20200722220519.404974280@linutronix.de