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 issue 8611.

LGTM=rsc
R=r, rsc, minux
CC=golang-codereviews
https://codereview.appspot.com/135990043

Committer: Russ Cox <rsc@golang.org>