math: faster Gamma

Having the compiler count the number of array elements speeds up Gamma from 63.7 to 56.6 ns/op.

R=rsc, golang-dev, r
CC=golang-dev
https://golang.org/cl/5362043

src/pkg/math/gamma.go

@@ -63,7 +63,7 @@ package math
 //   Stephen L. Moshier
 //   moshier@na-net.ornl.gov
 
-var _P = []float64{
+var _P = [...]float64{
 	1.60119522476751861407e-04,
 	1.19135147006586384913e-03,
 	1.04213797561761569935e-02,
@@ -72,7 +72,7 @@ var _P = []float64{
 	4.94214826801497100753e-01,
 	9.99999999999999996796e-01,
 }
-var _Q = []float64{
+var _Q = [...]float64{
 	-2.31581873324120129819e-05,
 	5.39605580493303397842e-04,
 	-4.45641913851797240494e-03,
@@ -82,7 +82,7 @@ var _Q = []float64{
 	7.14304917030273074085e-02,
 	1.00000000000000000320e+00,
 }
-var _S = []float64{
+var _S = [...]float64{
 	7.87311395793093628397e-04,
 	-2.29549961613378126380e-04,
 	-2.68132617805781232825e-03,