selftests: remove CROSS_COMPILE in dedicated Makefile

After previous clean up patches, memfd and timers could get
CROSS_COMPILE from tools/testing/selftest/lib.mk. There is no need to
preserve these definition. So, this patch remove them.

Acked-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Bamvor Jian Zhang <bamvor.zhangjian@linaro.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>