Use 16-byte stores instead of 8-byte stores to zero small blocks.
Also switch to duffzero for 65+ bytes only, because for each
duffzero call we also save/restore BP, so call requires 4 instructions
and replacing it with 4 sse stores doesn't cause code-bloat.
Also switch duffzero to use leaq, instead of addq to avoid clobbering flags.

ClearFat8-6     0.54ns ± 0%  0.54ns ± 0%     ~     (all equal)
ClearFat12-6    1.07ns ± 0%  1.07ns ± 0%     ~     (all equal)
ClearFat16-6    1.07ns ± 0%  0.69ns ± 0%  -35.51%  (p=0.001 n=8+9)
ClearFat24-6    1.61ns ± 1%  1.07ns ± 0%  -33.33%  (p=0.000 n=10+10)
ClearFat32-6    2.14ns ± 0%  1.07ns ± 0%  -50.00%  (p=0.001 n=8+9)
ClearFat40-6    2.67ns ± 1%  1.61ns ± 0%  -39.72%  (p=0.000 n=10+8)
ClearFat48-6    3.75ns ± 0%  2.68ns ± 0%  -28.59%  (p=0.000 n=9+9)
ClearFat56-6    4.29ns ± 0%  3.22ns ± 0%  -25.10%  (p=0.000 n=9+9)
ClearFat64-6    4.30ns ± 0%  3.22ns ± 0%  -25.15%  (p=0.000 n=8+8)
ClearFat128-6   7.50ns ± 1%  7.51ns ± 0%     ~     (p=0.767 n=10+9)
ClearFat256-6   13.9ns ± 1%  13.9ns ± 1%     ~     (p=0.257 n=10+10)
ClearFat512-6   26.8ns ± 0%  26.8ns ± 0%     ~     (p=0.467 n=8+8)
ClearFat1024-6  52.5ns ± 0%  52.5ns ± 0%     ~     (p=1.000 n=8+8)

Also shaves ~20kb from go tool:

go_old 10384994
go_new 10364514 [-20480 bytes]

section differences
global text (code) = -20585 bytes (-0.532047%)
read-only data = -302 bytes (-0.018101%)
Total difference -20887 bytes (-0.348731%)

Change-Id: I15854e87544545c1af24775df895e38e16e12694
Reviewed-on: https://go-review.googlesource.com/54410
Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>