cmd/go: fix spurious rebuild of binaries using cgo on OS X

The text segment starts farther into the binary when using
external linking on the mac. Test and fix.

Fixes #12173.

Change-Id: I1f0c81814bf70cd9decfceac3022784f4608eeef
Reviewed-on: https://go-review.googlesource.com/13672


Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>