[Arm64] Optimize Chacha20

This patch optimizes Chacha20 for arm64 architecture by following the approach used in powerpc implementation.

testsuite passes all tests of this patch.

Benchmark of chacha encrypt/decrypt using nettle-benchmark on gfarm 117

C	This patch
197.72 Mbyte/s	357.58 Mbyte/s

NOTE: This patch is implemented while both endianess modes are in mind but has been tested only on little-endian variant because of lack of big-endian access.