This allows one to customize which ar to use by fetching its path
from the environment. This way one can swap it out for a
different implementation.

Change-Id: I40d8cbd8a69e97b5254e66081d9bf0b726c10366
GitHub-Last-Rev: 4aa1d631eaca58be6c3a4be40f7404fa75a0db25
GitHub-Pull-Request: golang/go#28746
Reviewed-on: https://go-review.googlesource.com/c/149117
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>