math: math.Asinh returns incorrect result

[ALTree]

Related to #8909

math.Asinh(0.9037494326371905)
// go gives    :  0.8116512703034308
// c and python:  0.8116512703034306
// real value  :  0.81165127030343061483173...

http://play.golang.org/p/xw-OtkMuN2

[rsc]

We've never guaranteed to get bit-precise answers here. This is remarkably good actually.