test: gccgo and gc print the error on different lines.

This introduces GC_ERROR to mark an error only issued by the
gc compiler.  GCCGO_ERROR already exists to mark errors only
issued by the gccgo compiler.  Obviously these should be used
sparingly.

bug195.go:9: error: interface contains embedded non-interface
bug195.go:12: error: interface contains embedded non-interface
bug195.go:15: error: interface contains embedded non-interface
bug195.go:18: error: invalid recursive interface
bug195.go:22: error: invalid recursive interface

R=rsc
CC=golang-dev
https://golang.org/cl/2040043

test/errchk

@@ -60,8 +60,8 @@ sub bug() {
 $line = 0;
 foreach $src (@src) {
 	$line++;
-	next unless $src =~ m|// ERROR (.*)|;
-	$regexp = $1;
+	next unless $src =~ m|// (GC_)?ERROR (.*)|;
+	$regexp = $2;
 	if($regexp !~ /^"([^"]*)"/) {
 		print STDERR "$file:$line: malformed regexp\n";
 		next;

test/fixedbugs/bug195.go

@@ -19,9 +19,9 @@ type I4 interface {
 }
 
 type I5 interface {
-	I6
+	I6	// GCCGO_ERROR "interface"
 }
 
 type I6 interface {
-	I5	// ERROR "interface"
+	I5	// GC_ERROR "interface"
 }