selftests: vDSO: unconditionally build getrandom test

Rather than building on supported archs, build on all archs, and then
use the presence of the symbol in the vDSO to either skip the test or
move forward with it.

Note that this means that this test no longer checks whether the symbol
was correctly added to the kernel. But hopefully this will be clear
enough to developers and we'll cross our fingers that symbols aren't
removed by accident and not caught after this change.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>