selftests: vDSO: skip getrandom test if architecture is unsupported

If the getrandom test compiles for an arch, don't exit fatally if the
actual cpu it's running on is unsupported.

Suggested-by: Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>