selftest/x86: have no dependency on all when cross building

If the CROSS_COMPILE is set remove all's dependency on all_32 and all_64.

Cc: Andy Lutomirski <luto@amacapital.net>
Acked-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Tyler Baker <tyler.baker@linaro.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>