cmd/compile: use generated loops instead of DUFFZERO on amd64

cmd/compile: use generated loops instead of DUFFZERO on amd64

goarch: amd64
cpu: 12th Gen Intel(R) Core(TM) i7-12700
                        │     base      │                 exp                 │
                        │    sec/op     │   sec/op     vs base                │
MemclrKnownSize112-20      1.270n ± 14%   1.006n ± 0%  -20.72% (p=0.000 n=10)
MemclrKnownSize128-20      1.266n ±  0%   1.005n ± 0%  -20.58% (p=0.000 n=10)
MemclrKnownSize192-20      1.771n ±  0%   1.579n ± 1%  -10.84% (p=0.000 n=10)
MemclrKnownSize248-20      4.034n ±  0%   3.520n ± 0%  -12.75% (p=0.000 n=10)
MemclrKnownSize256-20      2.269n ±  0%   2.014n ± 0%  -11.26% (p=0.000 n=10)
MemclrKnownSize512-20      4.280n ±  0%   4.030n ± 0%   -5.84% (p=0.000 n=10)
MemclrKnownSize1024-20     8.309n ±  1%   8.057n ± 0%   -3.03% (p=0.000 n=10)

Change-Id: I8f1627e2a1e981ff351dc7178932b32a2627f765
Reviewed-on: https://go-review.googlesource.com/c/go/+/678937
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>