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>
5 files changed