math: speed up Gamma(+Inf)

Add special case for Gamma(+∞) which speeds it up:

benchmark            old ns/op     new ns/op     delta
BenchmarkGamma-4     14.5          7.44          -48.69%

The documentation for math.Gamma already specifies it as a special
case:

        Gamma(+Inf) = +Inf

The original C code that has been used as the reference implementation
(as mentioned in the comments in gamma.go) also treats Gamma(+∞) as a
special case:

if( x == INFINITY )
        return(x);

Change-Id: Idac36e19192b440475aec0796faa2d2c7f8abe0b
Reviewed-on: https://go-review.googlesource.com/31370


Reviewed-by: Robert Griesemer <gri@golang.org>