selftest/x86: build both bitnesses

Using uname with the processor flag option in some cases can yield 'unknown'
so lets use the machine flag option as it is deterministic. Add a dependency
for all_32 when building on a x86 64 bit host so that both bitnesses are
built in this case.

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>