cmd/go: do not require a valid archChar when using -compiler gccgo
cmd/go: do not require a valid archChar when using -compiler gccgo

Fixes issue 7152.

R=iant, rsc, gobot
CC=golang-codereviews
https://codereview.appspot.com/54790044

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