Documentation/vDSO: don't build 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/vDSO/vdso_standalone_test_x86.c:49:2: error: impossible
register constraint in 'asm'
make[4]: *** [Documentation/vDSO/vdso_standalone_test_x86.o] Error 1

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: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>