math/big: improve performance of addVW/subVW for ppc64x

math/big: improve performance of addVW/subVW for ppc64x

This change adds a better implementation in asm for addVW/subVW for
ppc64x, with speedups up to 3.11x.

benchmark                    old ns/op     new ns/op     delta
BenchmarkAddVW/1-16          6.87          5.71          -16.89%
BenchmarkAddVW/2-16          7.72          5.94          -23.06%
BenchmarkAddVW/3-16          8.74          6.56          -24.94%
BenchmarkAddVW/4-16          9.66          7.26          -24.84%
BenchmarkAddVW/5-16          10.8          7.26          -32.78%
BenchmarkAddVW/10-16         17.4          9.97          -42.70%
BenchmarkAddVW/100-16        164           56.0          -65.85%
BenchmarkAddVW/1000-16       1638          524           -68.01%
BenchmarkAddVW/10000-16      16421         5201          -68.33%
BenchmarkAddVW/100000-16     165762        53324         -67.83%
BenchmarkSubVW/1-16          6.76          5.62          -16.86%
BenchmarkSubVW/2-16          7.69          6.02          -21.72%
BenchmarkSubVW/3-16          8.85          6.61          -25.31%
BenchmarkSubVW/4-16          10.0          7.34          -26.60%
BenchmarkSubVW/5-16          11.3          7.33          -35.13%
BenchmarkSubVW/10-16         19.5          18.7          -4.10%
BenchmarkSubVW/100-16        153           55.9          -63.46%
BenchmarkSubVW/1000-16       1502          519           -65.45%
BenchmarkSubVW/10000-16      15005         5165          -65.58%
BenchmarkSubVW/100000-16     150620        53124         -64.73%

benchmark                    old MB/s     new MB/s     speedup
BenchmarkAddVW/1-16          1165.12      1400.76      1.20x
BenchmarkAddVW/2-16          2071.39      2693.25      1.30x
BenchmarkAddVW/3-16          2744.72      3656.92      1.33x
BenchmarkAddVW/4-16          3311.63      4407.34      1.33x
BenchmarkAddVW/5-16          3700.52      5512.48      1.49x
BenchmarkAddVW/10-16         4605.63      8026.37      1.74x
BenchmarkAddVW/100-16        4856.15      14296.76     2.94x
BenchmarkAddVW/1000-16       4883.96      15264.21     3.13x
BenchmarkAddVW/10000-16      4871.52      15380.78     3.16x
BenchmarkAddVW/100000-16     4826.17      15002.48     3.11x
BenchmarkSubVW/1-16          1183.20      1423.03      1.20x
BenchmarkSubVW/2-16          2081.92      2657.44      1.28x
BenchmarkSubVW/3-16          2711.52      3632.30      1.34x
BenchmarkSubVW/4-16          3198.30      4360.30      1.36x
BenchmarkSubVW/5-16          3534.43      5460.40      1.54x
BenchmarkSubVW/10-16         4106.34      4273.51      1.04x
BenchmarkSubVW/100-16        5213.48      14306.32     2.74x
BenchmarkSubVW/1000-16       5324.27      15391.21     2.89x
BenchmarkSubVW/10000-16      5331.33      15486.57     2.90x
BenchmarkSubVW/100000-16     5311.35      15059.01     2.84x

Change-Id: Ibaa5b9b38d63fba8e01a9c327eb8bef1e6e908c1
Reviewed-on: https://go-review.googlesource.com/101975
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>