cmd/gc: silence assignment errors to undefined symbols

Fixes #6406.

R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/46900043
diff --git a/test/typecheck.go b/test/typecheck.go
index a2ad91f..6f12042 100644
--- a/test/typecheck.go
+++ b/test/typecheck.go
@@ -14,5 +14,5 @@
 
 func main() {
 	mine()		// GCCGO_ERROR "not enough arguments"
-	c = mine()	// ERROR "undefined.*c|not enough arguments" "cannot assign to c"
+	c = mine()	// ERROR "undefined.*c|not enough arguments"
 }