cmd/6g, runtime: improve duffzero throughput

cmd/6g, runtime: improve duffzero throughput

It is faster to execute

	MOVQ AX,(DI)
	MOVQ AX,8(DI)
	MOVQ AX,16(DI)
	MOVQ AX,24(DI)
	ADDQ $32,DI

than

	STOSQ
	STOSQ
	STOSQ
	STOSQ

However, in order to be able to jump into
the middle of a block of MOVQs, the call
site needs to pre-adjust DI.

If we're clearing a small area, the cost
of that DI pre-adjustment isn't repaid.

This CL switches the DUFFZERO implementation
to use a hybrid strategy, in which small
clears use STOSQ as before, but large clears
use mostly MOVQ/ADDQ blocks.

benchmark                 old ns/op     new ns/op     delta
BenchmarkClearFat8        0.55          0.55          +0.00%
BenchmarkClearFat12       0.82          0.83          +1.22%
BenchmarkClearFat16       0.55          0.55          +0.00%
BenchmarkClearFat24       0.82          0.82          +0.00%
BenchmarkClearFat32       2.20          1.94          -11.82%
BenchmarkClearFat40       1.92          1.66          -13.54%
BenchmarkClearFat48       2.21          1.93          -12.67%
BenchmarkClearFat56       3.03          2.20          -27.39%
BenchmarkClearFat64       3.26          2.48          -23.93%
BenchmarkClearFat72       3.57          2.76          -22.69%
BenchmarkClearFat80       3.83          3.05          -20.37%
BenchmarkClearFat88       4.14          3.30          -20.29%
BenchmarkClearFat128      5.54          4.69          -15.34%
BenchmarkClearFat256      9.95          9.09          -8.64%
BenchmarkClearFat512      18.7          17.9          -4.28%
BenchmarkClearFat1024     36.2          35.4          -2.21%

Change-Id: Ic786406d9b3cab68d5a231688f9e66fcd1bd7103
Reviewed-on: https://go-review.googlesource.com/2585
Reviewed-by: Keith Randall <khr@golang.org>