This reduces the amount of duplication. Now there is only one list
of platforms supporting cgo.

Update #12270.

Change-Id: I5dcd55cb6be7c5bb6ce560383c71d90ab1189dc9
Reviewed-on: https://go-review.googlesource.com/14278
Run-TryBot: Minux Ma <minux@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>