Allow unaligned access on ARM64
Allow unaligned access on ARM64

Tested on a four-core Cortex-A53:

BenchmarkXOR-4                           4448383               270 ns/op        3796.09 MB/s
BenchmarkXORSafe-4                        589228              1997 ns/op         512.85 MB/s
BenchmarkXORFast-4                       4570785               262 ns/op        3905.32 MB/s

pion/stun,pion/stun