ARM64 copyinout improvements

The first of set of patches.
Use wider load/stores when aligned buffer is being copied.

In a simple test:

dd if=/dev/zero of=/dev/null bs=1M count=1024

the performance jumped from 410MB/s up to 3.6GB/s.

TODO:

• better handling of unaligned buffers (WiP)
• implement similar mechanism to bzero

Submitted by: Dominik Ermel <der@semihalf.com>
Obtained from: Semihalf
Sponsored by: Cavium
Reviewed by: kib, andrew, emaste
Differential Revision: https://reviews.freebsd.org/D5664