selftests/kcmp: Always try to build the test

Don't prevent the test building on non-x86. Just try and build it and
let the chips fall where they may.

Add support for CROSS_COMPILE while we're at it. Also we don't need a
custom rule for building kcmp_test.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Christopher Covington <cov@codeaurora.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>