selftests: vdso: Add a selftest for vDSO getcpu()

Provide a very basic selftest for getcpu() which similarly to our existing
test for gettimeofday() looks up the function in the vDSO and prints the
results it gets if the function exists and succeeds.

Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>