Documentation: Restrict TSC test code to x86

The prctl test code in Documentation/ tries to show how to
use a call that only makes sense on x86. Restrict it there
so that other platforms don't try to call asm("rdtsc").

Signed-off-by: Alexander Graf <agraf@suse.de>
Acked-by: Peter Foley <pefoley2@pefoley.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>