openssl: fix builds on ARMv8 targets without Aarch64

ARMv8 doesn't imply Aarch64, so correct a check that was making that
assumption.  This fixes the build on 32-bit ARMv8 targets such as
Cortex-A32.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>