Independent of the underlying CPU architecture, casting unaligned
pointers to uint64_t* is undefined. Just use a memcpy. The compiler
should be able to optimize that itself.

Change-Id: I39210871fca3eaf1f4b1d205b2bb0c337116d9cc
Reviewed-on: https://boringssl-review.googlesource.com/c/34872
Reviewed-by: Adam Langley <agl@google.com>