math/big: handle negative exponents in Exp

math/big: handle negative exponents in Exp

For modular exponentiation, negative exponents can be handled using
the following relation.

for y < 0: x**y mod m == (x**(-1))**|y| mod m

First compute ModInverse(x, m) and then compute the exponentiation
with the absolute value of the exponent.  Non-modular exponentiation
with a negative exponent still returns 1.

Fixes #25865

Change-Id: I2a35986a24794b48e549c8de935ac662d217d8a0
Reviewed-on: https://go-review.googlesource.com/118562
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>