examples: convert nettle-benchmark to use openssl EVP APIs

The nettle-benchmark program currently uses the openssl low level
cipher APIs for benchmarking. This means it always runs the generic
software implementation, never able to take advantage of impls
optimized for new hardware (eg AES-NI).

Rewriting it to use the higher EVP APIs means we can use the same
code for all ciphers, and automatically trigger hardware optimized
versions, giving a fairer comparison against openssl as commonly
used in applications.

Use of the generic openssl impl can still be forced by setting an
env variable  OPENSSL_ia32cap="~0x200000200000000"

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>