This change updates the ChaCha20-Poly1305 AEAD to be able to process
|extra_in| data. It does this by encrypting the extra data byte-by-byte
(because extra data should be very small). Both the generic and assembly
code is updated to be able to include this extra ciphertext in the
Poly1305 calculation.

Change-Id: I751ed31fb7e1f4db6974e9ed31721a43177cf8cb
Reviewed-on: https://boringssl-review.googlesource.com/17465
Reviewed-by: Adam Langley <agl@google.com>
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
Commit-Queue: Adam Langley <agl@google.com>