math/big: math.Exp should return result >= 0 for |m| > 0

The documentation states that Exp(x, y, m)
computes x**y mod |m| for m != nil && m > 0.
In math.big, Mod is the Euclidean modulus,
which is always >= 0.

Fixes issue 8822.

LGTM=agl, r, rsc
R=agl, r, rsc
CC=golang-codereviews
https://codereview.appspot.com/145650043