selftests/memfd: Run test on all architectures

Remove the dependence on x86 to run the memfd test. Verfied on 32-bit powerpc.

Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>