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
exp/gotype: disable broken test

R=golang-dev, iant
CC=golang-dev
https://codereview.appspot.com/7275045