math: use portable Exp instead of 387 instructions on 386

math: use portable Exp instead of 387 instructions on 386

The 387 implementation is less accurate and slower.

name     old time/op  new time/op  delta
Exp-8    29.7ns ± 2%  24.0ns ± 2%  -19.08%  (p=0.000 n=10+10)

This makes Gamma more accurate too.

Change-Id: Iad33b9cce0b087ccbce3e08ba7a6d285c4999d02
Reviewed-on: https://go-review.googlesource.com/30230
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Quentin Smith <quentin@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>