[release-branch.go1.5] math/big: fix Exp(x, x, x) for certain large x

Fixes #13907.

Change-Id: Ieaa5183f399b12a9177372212adf481c8f0b4a0d
Reviewed-on: https://go-review.googlesource.com/18491


Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: Vlad Krasnov <vlad@cloudflare.com>
Reviewed-by: Adam Langley <agl@golang.org>
Reviewed-on: https://go-review.googlesource.com/18586


Reviewed-by: Chris Broadfoot <cbro@golang.org>