math: improve Atan, Asin and Acos accuracy
pkg/math/all_test.go tests Atan (and therefore Asin and Acos) to a
relative accuracy of 4e-16, but the test vector misses values where
the old algorithm was in error by more than that. For example:
x newError oldError
0.
414215746 1.41e-16 -4.24e-16
0.
414216076 1.41e-16 -4.24e-16
0.
414217632 1.41e-16 -4.24e-16
0.
414218770 1.41e-16 -4.24e-16
0.
414225466 0 -5.65e-16
0.
414226244 1.41e-16 -4.24e-16
0.
414228756 0 -5.65e-16
0.
414235089 0 -5.65e-16
0.
414237070 0 -5.65e-16
R=rsc, golang-dev
CC=golang-dev
https://golang.org/cl/
6302093