Go: Performance optimization for addVV for Power

Go: Performance optimization for addVV for Power

I added a new implementation for addVV (math/big package) for Power architecture. The new assembly implementation leverages specific Power instructions and provides a speedup of ~3x over the generic implementation in Go. This works on both Little Endian and Big Endian ppc64, and will be available in the next go1.9 release.

In addition, for go1.10, I plan to add optimizations for math/big using POWER9 instructions, which will help some of the multiply-and-add functions.

Committed as 9459c03.

by Carlos Eduardo Seo