Always write the Poly1305 tag to an aligned buffer.

With GCC 4.9 and -O2 (and only -O2, -O1 and -O3 didn't trigger it), the
Poly1305 code can end up writing to an unaligned address otherwise and
that triggers a bus error on ARM.

Change-Id: Ifbeb7e2066a893d91d6f63c6565bac7d5542ef81
Reviewed-on: https://boringssl-review.googlesource.com/2850
Reviewed-by: Adam Langley <agl@google.com>