Don't require the ChaCha nonce to be aligned on ARM.

Change-Id: I34ee66fcc53d3371591beee3373c46598c31b5c5
Reviewed-on: https://boringssl-review.googlesource.com/3460
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>