Re-add the C version (only) of |EVP_aes_256_xts| Change-Id: I63c70f93a0f9395673c9fbe01eb5d864a14a48b6 Reviewed-on: https://boringssl-review.googlesource.com/5520 Reviewed-by: Adam Langley <agl@google.com> Reviewed-by: Matt Braithwaite <mab@google.com>