cmd/compile: use generated loops instead of DUFFZERO on riscv64

cmd/compile: use generated loops instead of DUFFZERO on riscv64

MemclrKnownSize112-4          5.602Gi ± 0%    5.601Gi ± 0%         ~ (p=0.363 n=10)
MemclrKnownSize128-4          6.933Gi ± 1%    6.545Gi ± 1%    -5.59% (p=0.000 n=10)
MemclrKnownSize192-4          8.055Gi ± 1%    7.804Gi ± 0%    -3.12% (p=0.000 n=10)
MemclrKnownSize248-4          8.489Gi ± 0%    8.718Gi ± 0%    +2.69% (p=0.000 n=10)
MemclrKnownSize256-4          8.762Gi ± 0%    8.763Gi ± 0%         ~ (p=0.494 n=10)
MemclrKnownSize512-4          9.514Gi ± 1%    9.514Gi ± 0%         ~ (p=0.529 n=10)
MemclrKnownSize1024-4         9.940Gi ± 0%    9.939Gi ± 1%         ~ (p=0.989 n=10)
ClearFat3-4                   1.300Gi ± 0%    1.301Gi ±  0%         ~ (p=0.447 n=10)
ClearFat4-4                   3.902Gi ± 0%    3.902Gi ±  0%         ~ (p=0.971 n=10)
ClearFat5-4                   665.8Mi ± 0%   1331.5Mi ±  0%  +100.01% (p=0.000 n=10)
ClearFat6-4                   665.8Mi ± 0%   1330.5Mi ±  0%   +99.82% (p=0.000 n=10)
ClearFat7-4                   490.7Mi ± 0%   1331.9Mi ±  0%  +171.45% (p=0.000 n=10)
ClearFat8-4                   5.201Gi ± 0%    5.202Gi ±  0%         ~ (p=0.123 n=10)
ClearFat9-4                   856.1Mi ± 0%   1331.6Mi ±  0%   +55.54% (p=0.000 n=10)
ClearFat10-4                  887.8Mi ± 0%   1331.9Mi ±  0%   +50.03% (p=0.000 n=10)
ClearFat11-4                  915.3Mi ± 0%   1331.1Mi ±  0%   +45.42% (p=0.000 n=10)
ClearFat12-4                  5.202Gi ± 0%    5.202Gi ±  0%         ~ (p=0.481 n=10)
ClearFat13-4                  961.5Mi ± 0%   1331.8Mi ±  0%   +38.50% (p=0.000 n=10)
ClearFat14-4                  981.0Mi ± 0%   1331.8Mi ±  0%   +35.76% (p=0.000 n=10)
ClearFat15-4                  951.3Mi ± 0%   1331.4Mi ±  0%   +39.96% (p=0.000 n=10)
ClearFat16-4                  1.600Gi ± 0%    5.202Gi ±  0%  +225.10% (p=0.000 n=10)
ClearFat18-4                  1.018Gi ± 0%    1.300Gi ±  0%   +27.77% (p=0.000 n=10)
ClearFat20-4                  2.601Gi ± 0%    4.938Gi ± 12%   +89.87% (p=0.000 n=10)
ClearFat24-4                  2.601Gi ± 0%    5.201Gi ±  0%   +99.96% (p=0.000 n=10)
ClearFat32-4                  1.982Gi ± 0%    5.203Gi ±  0%  +162.55% (p=0.000 n=10)
ClearFat40-4                  3.467Gi ± 0%    4.338Gi ±  0%   +25.11% (p=0.000 n=10)
ClearFat48-4                  3.671Gi ± 0%    5.201Gi ±  0%   +41.69% (p=0.000 n=10)
ClearFat56-4                  3.640Gi ± 0%    5.201Gi ±  0%   +42.88% (p=0.000 n=10)
ClearFat64-4                  2.250Gi ± 0%    5.202Gi ±  0%  +131.25% (p=0.000 n=10)
ClearFat72-4                  4.064Gi ± 0%    5.201Gi ±  0%   +27.97% (p=0.000 n=10)
ClearFat128-4                 4.496Gi ± 0%    5.203Gi ±  0%   +15.71% (p=0.000 n=10)
ClearFat256-4                 4.756Gi ± 0%    5.201Gi ±  0%    +9.36% (p=0.000 n=10)
ClearFat512-4                 2.512Gi ± 0%    5.201Gi ±  0%  +107.03% (p=0.000 n=10)
ClearFat1024-4                4.255Gi ± 0%    5.202Gi ±  0%   +22.26% (p=0.000 n=10)
ClearFat1032-4                4.260Gi ± 0%    5.201Gi ±  0%   +22.09% (p=0.000 n=10)
ClearFat1040-4                4.285Gi ± 1%    5.203Gi ±  0%   +21.41% (p=0.000 n=10)
geomean                       2.005Gi         3.020Gi         +50.58%

Change-Id: Iea1da734ff8eaf1b5a2822ae2bdb7f4fd9b65651
Reviewed-on: https://go-review.googlesource.com/c/go/+/699635
Reviewed-by: Mark Ryan <markdryan@rivosinc.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Mark Freeman <markfreeman@google.com>