math: improve sqrt for ppc64le,ppc64

math: improve sqrt for ppc64le,ppc64

The existing implementation uses code written in Go to
implement Sqrt; this adds the assembler to use the sqrt
instruction for Power and makes the necessary changes to
allow it to be inlined.

The following tests showed this relative improvement:

benchmark                 delta
BenchmarkSqrt             -97.91%
BenchmarkSqrtIndirect     -96.65%
BenchmarkSqrtGo           -35.93%
BenchmarkSqrtPrime        -96.94%

Fixes #14349

Change-Id: I8074f4dc63486e756587564ceb320aca300bf5fa
Reviewed-on: https://go-review.googlesource.com/19515
Reviewed-by: Minux Ma <minux@golang.org>