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

gnanderson/go,tv42/old-go,pombredanne/go-deleteme,gnanderson/go,tv42/old-go,gnanderson/go,apparentlymart/golang,dsriram/golang,pombredanne/go-deleteme,tv42/old-go,gnanderson/go,pombredanne/go-deleteme,apparentlymart/golang,tv42/old-go,apparentlymart/golang,dsriram/golang,gnanderson/go,dsriram/golang,tv42/old-go,dsriram/golang,dsriram/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,apparentlymart/golang,dsriram/golang,apparentlymart/golang,pombredanne/go-deleteme,gnanderson/go,tv42/old-go,pombredanne/go-deleteme,gnanderson/go,dsriram/golang,tv42/old-go,apparentlymart/golang,apparentlymart/golang