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