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: NAlexander Graf <agraf@suse.de>
Acked-by: NPeter Foley <pefoley2@pefoley.com>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/prctl/Makefile

 # List of programs to build
-hostprogs-y := disable-tsc-ctxt-sw-stress-test disable-tsc-on-off-stress-test disable-tsc-test
+hostprogs-$(CONFIG_X86) := disable-tsc-ctxt-sw-stress-test disable-tsc-on-off-stress-test disable-tsc-test
 # Tell kbuild to always build the programs
 always := $(hostprogs-y)