cmd/vet: infer asm arch from build context

If we cannot infer the asm arch from the filename
or the build tags, assume that it is the
current build arch. Assembly files with no
restrictions ought to be usable on all arches.

Updates #11041

Change-Id: I0ae807dbbd5fb67ca21d0157fe180237a074113a
Reviewed-on: https://go-review.googlesource.com/27151


Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>