runtime, cmd/compile: use ldp for DUFFCOPY on ARM64

runtime, cmd/compile: use ldp for DUFFCOPY on ARM64

name         old time/op  new time/op  delta
CopyFat8     2.15ns ± 1%  2.19ns ± 6%     ~     (p=0.171 n=8+9)
CopyFat12    2.15ns ± 0%  2.17ns ± 2%     ~     (p=0.137 n=8+10)
CopyFat16    2.17ns ± 3%  2.15ns ± 0%     ~     (p=0.211 n=10+10)
CopyFat24    2.16ns ± 1%  2.15ns ± 0%     ~     (p=0.087 n=10+10)
CopyFat32    11.5ns ± 0%  12.8ns ± 2%  +10.87%  (p=0.000 n=8+10)
CopyFat64    20.2ns ± 2%  12.9ns ± 0%  -36.11%  (p=0.000 n=10+10)
CopyFat128   37.2ns ± 0%  21.5ns ± 0%  -42.20%  (p=0.000 n=10+10)
CopyFat256   71.6ns ± 0%  38.7ns ± 0%  -45.95%  (p=0.000 n=10+10)
CopyFat512    140ns ± 0%    73ns ± 0%  -47.86%  (p=0.000 n=10+9)
CopyFat520    142ns ± 0%    74ns ± 0%  -47.54%  (p=0.000 n=10+10)
CopyFat1024   277ns ± 0%   141ns ± 0%  -49.10%  (p=0.000 n=10+10)

Change-Id: If54bc571add5db674d5e081579c87e80153d0a5a
Reviewed-on: https://go-review.googlesource.com/97395
Reviewed-by: Cherry Zhang <cherryyz@google.com>