Documentation/prctl: don't build tsc tests when cross compiling

The following was seen in linux-next build coverage, which is somewhat
unique since it uses powerpc host to cross compile x86:

Documentation/prctl/disable-tsc-on-off-stress-test.c:36:1: error:
 impossible register constraint in 'asm'
Documentation/prctl/disable-tsc-ctxt-sw-stress-test.c:34:1: error:
 impossible register constraint in 'asm'
Documentation/prctl/disable-tsc-test.c:36:1: error: impossible
 register constraint in 'asm'

It probably makes sense to just skip building these tests when
we are cross compiling.

Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Cc: Erik Bosman <ejbosman@cs.vu.nl>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>