math/big: ppc64 has no assembly implementation of arith.go functions

[izzystardust]

Please answer these questions before submitting your issue. Thanks!

1. What version of Go are you using (go version)?
   go version devel +b882707 Thu Jul 21 15:33:00 2016 -0500 linux/ppc64le
2. What operating system and processor architecture are you using (go env)?
   linux/ppc64le
3. What did you do?
   I noticed that performance of crypto/ecdsa on POWER was significantly slower than on Intel, due in large part to time spent in functions in arith.go
4. What did you expect to see?
   Similar performance profile to Intel.

I have a change that optimizes hotspots in ppc64 assembly.

[bradfitz]

/cc @laboger

[gopherbot]

CL https://golang.org/cl/26951 mentions this issue.