misc/cgo/test: disable issue 8428 regress test on darwin

Clang 3.2 and older (as shipped with OS X Mountain Lion and older)
outputs ambiguous DWARF debug info that makes it impossible for us to
reconstruct accurate type information as required for this test.

Fixes golang#8611.

LGTM=rsc
R=r, rsc, minux
CC=golang-codereviews
https://golang.org/cl/135990043

misc/cgo/test/issue8428.go

@@ -2,6 +2,10 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// This test fails on older versions of OS X because they use older buggy
+// versions of Clang that emit ambiguous DWARF info.  See issue 8611.
+// +build !darwin
+
 package cgotest
 
 // Issue 8428.  Cgo inconsistently translated zero size arrays.