The test program requires static constructor, which in turn needs
external linking to work, but external linking never works on 10.6.

This should fix the darwin-{386,amd64} builders.

Change-Id: I714fdd3e35f9a7e5f5659cf26367feec9412444f
Reviewed-on: https://go-review.googlesource.com/2235Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>