math/big: improve performance for addVV/subVV for ppc64x

math/big: improve performance for addVV/subVV for ppc64x

This change adds a better asm implementation of addVV for ppc64x, with speedups
up to nearly 3x in the best cases.

benchmark                   old ns/op     new ns/op     delta
BenchmarkAddVV/1-8          7.33          5.81          -20.74%
BenchmarkAddVV/2-8          8.72          6.49          -25.57%
BenchmarkAddVV/3-8          10.5          7.08          -32.57%
BenchmarkAddVV/4-8          12.7          7.57          -40.39%
BenchmarkAddVV/5-8          14.3          8.06          -43.64%
BenchmarkAddVV/10-8         27.6          11.1          -59.78%
BenchmarkAddVV/100-8        218           82.4          -62.20%
BenchmarkAddVV/1000-8       2064          718           -65.21%
BenchmarkAddVV/10000-8      20536         7153          -65.17%
BenchmarkAddVV/100000-8     211004        72403         -65.69%

benchmark                   old MB/s     new MB/s     speedup
BenchmarkAddVV/1-8          8729.74      11006.26     1.26x
BenchmarkAddVV/2-8          14683.65     19707.55     1.34x
BenchmarkAddVV/3-8          18226.96     27103.63     1.49x
BenchmarkAddVV/4-8          20204.50     33805.81     1.67x
BenchmarkAddVV/5-8          22348.64     39694.06     1.78x
BenchmarkAddVV/10-8         23212.74     57631.08     2.48x
BenchmarkAddVV/100-8        29300.07     77629.53     2.65x
BenchmarkAddVV/1000-8       31000.56     89094.54     2.87x
BenchmarkAddVV/10000-8      31163.61     89469.16     2.87x
BenchmarkAddVV/100000-8     30331.16     88393.73     2.91x

It also adds the use of CTR for the loop counter in subVV, instead of
manually updating the loop counter. This is slightly faster.

Change-Id: Ic4b05cad384fd057972d46a5618ed5c3039d7460
Reviewed-on: https://go-review.googlesource.com/41010
Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>