Following CL 68150047, the goos and goarch
variables are not currently set when the GOOS
and GOARCH environment variables are not set.

This made the content of the build tag to be
ignored in this case.

This CL sets goos and goarch to runtime.GOOS
and runtime.GOARCH when the GOOS and GOARCH
environments variables are not set.

LGTM=aram, bradfitz
R=golang-codereviews, aram, gobot, rsc, dave, bradfitz
CC=golang-codereviews, rsc
https://golang.org/cl/112490043