cmd/gc: remove reference to ? and @ symbols in error message
cmd/gc: remove reference to ? and @ symbols in error message

Those symbols are only allowed during imports;
the parser may expect them but saying that doesn't help users.

Fixes issue 3434.

R=ken2
CC=golang-dev
https://codereview.appspot.com/7277045

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