[release-branch.go1.4] build: disable race external linking test on OS X 10.6 and earlier

««« CL 176070043 / 500cb52e08e6
build: disable race external linking test on OS X 10.6 and earlier

External linking doesn't work there at all.

LGTM=bradfitz
R=adg, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/176070043
»»»

LGTM=bradfitz, adg
R=adg, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/175400043