AES-GCM-SIV is an AEAD with nonce-misuse resistance. It can reuse
hardware support for AES-GCM and thus encrypt at ~66% the speed, and
decrypt at 100% the speed, of AES-GCM.

See https://tools.ietf.org/html/draft-irtf-cfrg-gcmsiv-02

This implementation is generic, not optimised, and reuses existing AES
and GHASH support as much as possible. It is guarded by !OPENSSL_SMALL,
at least for now.

Change-Id: Ia9f77b256ef5dfb8588bb9ecfe6ee0e827626f57
Reviewed-on: https://boringssl-review.googlesource.com/12541
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: Adam Langley <agl@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>