- changes tests to check that the real and imaginary part of the go complex
  division result is equal to the result gcc produces for c99
- changes complex division code to satisfy new complex division test
- adds float functions isNan, isFinite, isInf, abs and copysign
  in the runtime package

Fixes #14644.

name                   old time/op  new time/op  delta
Complex128DivNormal-4  21.8ns ± 6%  13.9ns ± 6%  -36.37%  (p=0.000 n=20+20)
Complex128DivNisNaN-4  14.1ns ± 1%  15.0ns ± 1%   +5.86%  (p=0.000 n=20+19)
Complex128DivDisNaN-4  12.5ns ± 1%  16.7ns ± 1%  +33.79%  (p=0.000 n=19+20)
Complex128DivNisInf-4  10.1ns ± 1%  13.0ns ± 1%  +28.25%  (p=0.000 n=20+19)
Complex128DivDisInf-4  11.0ns ± 1%  20.9ns ± 1%  +90.69%  (p=0.000 n=16+19)
ComplexAlgMap-4        86.7ns ± 1%  86.8ns ± 2%     ~     (p=0.804 n=20+20)

Change-Id: I261f3b4a81f6cc858bc7ff48f6fd1b39c300abf0
Reviewed-on: https://go-review.googlesource.com/37441Reviewed-by: Robert Griesemer <gri@golang.org>