Add GOOS=linux to vet_asm script to avoid errors on OS not working on
amd64 architecture.

Change-Id: Ic3db43618008ae8e8bc08738c95c200dd4e916d7
Reviewed-on: https://go-review.googlesource.com/c/149577Reviewed-by: Alan Donovan <adonovan@google.com>
Run-TryBot: Alan Donovan <adonovan@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>