cmd/api: ignore GOARCH when building cmd/api.
cmd/api: ignore GOARCH when building cmd/api.

This was breaking people setting GOARCH=386 before running
all.bash on amd64 machines.

cmd/go puts different architecture binaries where "go tool"
can't find them.

R=golang-dev, r, khr
CC=golang-dev
https://codereview.appspot.com/13139044

wesolows/golang-old,wesolows/golang,wesolows/golang,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old