While most of OpenSSL's assembly allows out < in too, some of it doesn't.
Upstream seems to not consider this a problem (or, at least, they're failing to
make a decision on whether it is a problem, so we should assume they'll stay
their course). Accordingly, require aliased buffers to exactly align so we
don't have to keep chasing this down.

Change-Id: I00eb3df3e195b249116c68f7272442918d7077eb
Reviewed-on: https://boringssl-review.googlesource.com/8231
Reviewed-by: Adam Langley <agl@google.com>

The coverage tool revealed that we weren't testing all codepaths of the ChaCha
assembly. Add a standalone test as it's much easier to iterate over all lengths
when there isn't the entire AEAD in the way.

I wasn't able to find a really long test vector, so I generated a random one
with the Go implementation we have in runner.

This test gives us full coverage on the ChaCha20_ssse3 variant. (We'll see how
it fares on the other codepaths when the multi-variant test harnesses get in. I
certainly hope there isn't a more novel way to call ChaCha20 than this...)

Change-Id: I087e421c7351f46ea65dacdc7127e4fbf5f4c0aa
Reviewed-on: https://boringssl-review.googlesource.com/7299
Reviewed-by: Adam Langley <agl@google.com>