kselftest: Enable vDSO test on non x86 platforms

Currently the vDSO tests are built only on x86 platforms and cannot be
cross compiled.

Enable vDSO TARGET for all the platforms.

Future patches will extend the tests.

Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>