cmd/compile: use arm64 neon in LoweredMemmove/LoweredMemmoveLoop

cmd/compile: use arm64 neon in LoweredMemmove/LoweredMemmoveLoop

Raspberry Pi 5 (Cortex-A76)

                     │   base.log   │               opt.log               │
                     │    sec/op    │   sec/op     vs base                │
MemmoveKnownSize112     3.549n ± 0%   3.652n ± 0%   +2.92% (p=0.000 n=10)
MemmoveKnownSize128     3.979n ± 0%   3.617n ± 0%   -9.09% (p=0.000 n=10)
MemmoveKnownSize192     7.566n ± 0%   5.074n ± 0%  -32.94% (p=0.000 n=10)
MemmoveKnownSize248     8.549n ± 0%   7.184n ± 1%  -15.97% (p=0.000 n=10)
MemmoveKnownSize256    10.010n ± 0%   6.827n ± 0%  -31.80% (p=0.000 n=10)
MemmoveKnownSize512     19.81n ± 0%   13.59n ± 0%  -31.40% (p=0.000 n=10)
MemmoveKnownSize1024    39.66n ± 0%   27.00n ± 0%  -31.93% (p=0.000 n=10)
geomean                 9.538n        7.392n       -22.50%

Change-Id: I7b17408cd0a500ceaa80bc93ffe2f19ddeea9c0d
Reviewed-on: https://go-review.googlesource.com/c/go/+/692315
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>