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 #3434.

R=ken2
CC=golang-dev
https://golang.org/cl/7277045

src/cmd/gc/subr.c

@@ -152,13 +152,22 @@ yyerror(char *fmt, ...)
 			return;
 		lastsyntax = lexlineno;
 			
-		if(strstr(fmt, "{ or {")) {
+		if(strstr(fmt, "{ or {") || strstr(fmt, " or ?") || strstr(fmt, " or @")) {
 			// The grammar has { and LBRACE but both show up as {.
 			// Rewrite syntax error referring to "{ or {" to say just "{".
 			strecpy(buf, buf+sizeof buf, fmt);
 			p = strstr(buf, "{ or {");
 			if(p)
 				memmove(p+1, p+6, strlen(p+6)+1);
+			
+			// The grammar has ? and @ but only for reading imports.
+			// Silence them in ordinary errors.
+			p = strstr(buf, " or ?");
+			if(p)
+				memmove(p, p+5, strlen(p+5)+1);
+			p = strstr(buf, " or @");
+			if(p)
+				memmove(p, p+5, strlen(p+5)+1);
 			fmt = buf;
 		}