If a build directive contains multiple arches,
try to match the build context.

Updates #11041

Change-Id: I03b5d7bfb29d1ff6c7d36a9d7c7fabfcc1d871c1
Reviewed-on: https://go-review.googlesource.com/27158
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>