selftests/vDSO: add tests for vgetrandom

This adds two tests for vgetrandom. The first one, vdso_test_chacha,
simply checks that the assembly implementation of chacha20 matches that
of libsodium, a basic sanity check that should catch most errors. The
second, vdso_test_getrandom, is a full "libc-like" implementation of the
userspace side of vgetrandom() support. It's meant to be used also as
example code for libcs that might be integrating this.

Cc: linux-kselftest@vger.kernel.org
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>